The Assay-book 


N 

550 


Appleton 







Class - / ! — 

Bookl4-i^— 


Copi^htW 


COPYRIGHT DEPOSIT. 

















N. 


The 


ASSAY-BOOK 

FOR 

STUDENTS 


BY 


JOHN HOWARD APPLETON, Sc. D. 

>1 


Professor of Chemistry in Brown University 



Providence, R, I. 
Remington Printing Company 
1906 


LIBRARY of CONGRESS 
Two CoDies Received 

m 9 1906 



COPYRIGHT 1906 
BY 

JOHN HOWARD APPLETON 






CONTENTS 


ZINC 

Principal ores, 1; synopsis of methods, 1. 

Assay of calamine, 2; the stock solution. 3; gravimetric test, oxide method, 3; 
volumetric test, ferrocyanide method, 4. 


LEAD 

Principal ores, G; synopsis of methods, 0. 

Assay of galena, 7; gravimetric test, sulphate method, 7; volumetric test, chromate 
method, 8; crucible test, by iron wires, 9. 

Experiments with lead, and lead oxide, 10. 

ANTIMONY 

Principal ore, 12; synopsis of methods, 12. 

Assay of stibnite, 13; gravimetric test, sulphide method, 13: vmlumetric test, iodine 
method, 14. 


TIN 

Principal ores, 17; synopsis of methods, 17. 

Assay of cassiterite, 18; volumetric test, dichromate method, 18; crucible test, by 
jjotassium cyanide, 20. 


COPPER 

Principal ores, 21; synopsis of methods, 22. 

Assay of chalcopyrite, 23; stock solution, 23; gravimetric test, electrolytic method, 
24; notes on the electrolytic precipitation of coiiper, 25; gravimetric test, sulphide 
method, 28; gravimetric test, oxide method, 29; volumetric test, potassium cyanide 
method, 29; crucible test, German method, 30. 

IRON 

Principal ores, 32; synopsis of methods, 32. 

Assay of limonite, 35; A, the stock solution, 35; B, gravimetric test for iron and 
aluminium, basic acetate method, 36; C, voluinetric lest, permanganate method, 37; D, 
volumetric test, dichromate method, 39; E, volumetric test, stannous chloride method, 40. 

GOLD AND SILVER 

General introduction, 44. 

GOLD 

Principal ores, 45; synopsis of methods, 45. 

SILVER 

Principal ores, 46; synopsis of methods, 47. 

Assay of silver bullion, chloride method, 48: volumetric test. Gay Lussac’s method, 
48; volumetric test, sulphocyanate method, 51; dry test, cupellation method, 52. 

GOLD AND SILVER TOGETHER 

Gold and silver in bullion, .54; .synops's of methods. 54. 

Gold and silver in ores, .54; synopsis of methods, 54. 

Assay of gold and silver bullion, wet process, 55; cupellation process, 56. 

Assay of gold and silver ore, crucible process, 57. 

Remarks on the crucible process, 60. 


Ill 


BOOKS FOR REFERENCE 


Assayings In General 

Beringer, C. & J. J.—A Text-Book of Assaying. Philadelphia, 1890. 

J. B. Lippincott Co. 

Hiorns, a. H.—Practical Metallurgy and Assaying. New York, 1888. 
The Macmillan Co. 

Brown, W. L.^—Manual of Assaying. Chicago, 1889. 

E. H. Sargent & Co. 

Ricketts, P. de P. and Miller, E. A.—Notes on Assaying. New York, 1897. 
J. Wiley & Sons. 


Iron, Steel, Etc. 

Blair, A. A.—The Chemical Analysis of Iron. 5th Ed. Philadelphia, 1902. 
J. B. Lippincott Co. 

IIioRNS, A. H.—Iron and Steel Manufacture. New York, 1889. 

Macmillan & Co. 

Williams, W. M.—The Chemistry of Iron and Steel Making, London, 1890. 
ChATTO & WiNDUS. 

Turner, T. —The Metallurgy of Iron. Philadelphia, 1895. 

J. B. Lippincott Co. 

Behrens, H.—Das Mikroskopische Gefiige der Metalle und Legierungen 
Leipzig, 1894. Leopold Voss. 

Mineralogy in General 

Dana, J. 1).—A System of Mineralogy. 6th Ed. New York, 1895. 

J. Wiley & Sons. 

Chester, A. H.—A Dictionary of the Names of Minerals. New York, 1896. 
J. Wiley & Sons. (Contains 4627 names.) 

Quantitative Analysis 

Olsen, J. C.—Quantitative Chemical Analysis. New York, 1904. 

D. Van Nostrand Co. 

Fresenius, C. R. (Cohn, A. I., tr.).—Quantitative Chemical Analysis. 

New York, 1904. John Wiley & Sons. 

Talbot, LI. P.—Quantitative Chemical Analysis. 4th Ed. New York, 1902. 
The MacMiLLAN Co. 

Sutton, F.—Volumetric Analysis. Philadelphia, 1900. 

P. Blakiston’s Son & Co. 

ScHiMPF, H. W.— Volumetric Analysis. New York, 1903. 

John Wiley & Sons. 

Hillebrand, W. F.—Some Principles and Methods of Rock Analysis. 

Bull. U. S. Geol. Sur., No. 176. Washington, 1900. 

(This is reprinted in Fres. Quant. Anal., 2; 1101.) 


IV 


ZINC 

Principal Ores of Zinc 

Sinithsonite, (Dana, 279), Calamine, (Dana, 546), 

Zinc carbonate. Hydrous silicate of zinc. 



ZnCOg. 


Zn 2 Si 04 .H 20 

CO 3 

35.2 

SiOa 

25.0 

ZnO 

64.88 (Zn, 52.1) 

ZnO 

67.5 (Zn, 54.2). 



H 3 O 

7.5 


100 . 


100 . 


Blende, black-jack, (Dana 59), 


Franklinite, (Dana, 227) 


Zinc sulphide. 


Metallic oxides. 


ZnS. 

(Fe,- 

Zn, Mn) 0. (Fe, Mn) 2 O; 



FCaOg 

60.52 

S 

33. 

MriaOg 

6.79 

Zn 

67. 

ZnO 

19.44 (Zn,15.6) 



MnO 

12.81 


100 . 


99.56 


Zincite, (Dana, 208), 
Zinc oxide, 
ZnO. 

O 19.7 

Zn 80.3 


100 . 


Zinc—Synopsis of Methods 

Gravimetric Methods 

1 . Zinc oxide. Fresenius, 1 : 286. 

a. By precipitation as zinc carbonate. 

Precipitate from a [soiling solution with sodium carbonate. 

Wash, dry, and ignite. Weigh as ZnO. 

b. By precipitation as zinc sulphide. 

Make the solution alkaline with ammonia and add ammonium 
chloride. 

Precipitate as sulphide with colorless ammonium sulphide. 

Dissolve in hydrochloric acid and re-precipitate as carbonate. 
Weigh as ZnO. 

c. By direct ignition. 

Heat the compound in a weighed platinum crucible, raising the 
temperature gradually to a very intense heat. Weigh as ZnO. 


1 










2 


THE ASS AY-BOOK 


2. Zinc sulphide, Fresenius, 1: 289. 

Precipitate as sulphide from an ammoniacal solution. 

Heat in a weighed crucible in a current of hydrogen, or of hydrogen 
sulphide, (or illuminating gas). Weigh as ZnS. 

3. Zinc ammonium phosphate, Talbot, 55. 

Precipitate with neutral ammonium phosphate in a neutral solution. 

AVeigh, after drying at 105° C. 

Volumetric Methods 

1. Indirect method. Mann’s method. Fresenius, 2: 444. 

Precipitate the zinc as hydrated sulphide. 

Decompose the zinc sulphide with moist silver chloride. 

To the zinc chloride (thus formed) add a definite volume of standard 
silver solution—but an excess. 

Determine the excess with standard ammonium sulphocyanate, 
using a ferric indicator. (Volhard’s method.) 

Subtract the excess from the whole amount of silver solution, and 
from the difference compute the amount of zinc chloride, and 
thence of zinc itself. 

2. Sulphide method. Schwarz’s method. Sutton 377. 

Precipitate the zinc as sulphide from an ammoniacal solution. 

Digest the precipitate with ferric sulphate and sulphuric acid, until 
only a slight yellow color from undecomposed ferric salt remains. 

Titrate the reduced iron compound with a standard permanganate 
solution (2 atoms of iron represent 1 atom of zinc). 

3. Ferrocyanide method. Sutton, 382. 

a. In acetic acid solution (without removal of iron), Galetti’s method. 

Convert the zinc into a soluble acetate. 

Titrate with standard ferrocyanide, using uranium acetate as indi¬ 
cator. 

b. In hydrochloric acid solution (requiring removal of iron). Fahl- 

berg’s method. 

Titrate with ferrocyanide solution, using uranium acetate as indi¬ 
cator. 


Dry Methods 

Since metallic zinc is volatile at comparatively low temperatures, it is 
hardly practicable to use dry processes. 


Zinc—Assay of Calamine 

A. —The stock solution. 

B. —Gravimetric test: oxide method, with preliminary precipitation as 
sulphide. 

C. —Volumetric test: ferrocyanide method 

(No dry process.) 




ZINC 


3 


A.—The Stock Solution 

The intention is to produce, from 10 gms. of ore, one stock solution of 
the volume of 1 litre. Next it is intended to use separate parts of this solu¬ 
tion for separate gravimetric and volumetric tests. 

Outline of the Process 

a. Treat the powdered ore with potassium chlorate and nitric acid. This 
action is intended to be solvent for zinc compounds and oxidizing for sulphides. 

Evaporate the solution to remove oxides of nitrogen and chlorine. 

b. Next treat with hydrochloric acid to dissolve zinc and to liberate silicic 

acid. 

Evaporate again and dry thoroughly to produce insoluble silicic oxide. 

c. Filter to remove silicic oxide. 

The Process 

1. Weighing and dissolving. — Weigh ten g. of the finely powdered 

ore. 

Transfer it to a No. 4 beaker. Add 50 cc. of water, and 5 g. of po¬ 
tassium chlorate. Next add 10 cc. of nitric acid. Cover the beaker. Warm 
the mixture for 30 minutes on an iron plate. 

Evaporate to dryness, in the beaker, and on the plate. 

To the residue, add 20 cc. of water and 30 cc. of'hydrochloric acid. 

2. Removing silicic oxide. —Evaporate again upon the plate to complete 
dryness (in order to change the gelatinous silicic acid to insoluble silicic 
oxide). 

Cool the dry mass; then add to it 50 cc. of water and 10 cc. of hydro¬ 
chloric acid. Warm the mixture, to dissolve the zinc (but leaving silicic an¬ 
hydride insoluble). Heat the mixture to boiling. 

Filter with the filter-pump and wash. (Dry and ignite the paper and 
contents; weigh the residue as silica.) 

3. Completing the stock solution. —Transfer the filtrate from the pre¬ 
ceding process to a litre flask; dilute the whole to 1000 cc.; 100 cc. of it repre¬ 
sents 1 g. of the original ore. 

B.—Gravimetric Method for Zinc* 

Oxide method, with preliminary precipitation as sulphide. 

Beringer, 217. 

Outline of the Process 

Precipitate the zinc first as sulphide, then as basic carbonate. 

Weigh as zinc oxide, ZnO. 


Supplies 

Apparatus for generating hydrogen sulphide. 




4 


THE ASSAY BOOK 


The Process 

1. Drawing the sample. —Draw from the stock solution 100 cc. (It 
represents 1 g. of ore.) To this, add 100 cc. of water. 

2. Removing copper, etc. —Saturate the solution with hydrogen sulphide; 
filter; wash with water acidulated with hydrochloric acid. Boil to expel 
hydrogen sulphide; add a few drops of nitric acid. Cool. 

3. Removing iron and aluminium. —Add caustic soda (about 10 cc.) until 
the acid is nearly neutralized and then sodium acetate (10 g. in 20 cc. of 
water) until the color of the solution is no longer darkened. Boil. Filter. 

4. Separating zinc. —To the filtrate add ammonia (about 10 cc.) until 
the solution is alkaline; then pass hydrogen sulphide in. Allow the precipi¬ 
tate to subside; decant upon ^ filter. . 

Remove the filter paper with the precipitate to a beaker and add 20 cc. 
of concentrated hydrochloric acid and 20 cc. of water. Boil to free from 
hydrogen sulphide and evaporate, if necessary, to get rid of excess of acid. 

The solution should contain the zinc (together with any manganese the 
ore contained) and perhaps traces of cobalt and nickel. Evaporate this so¬ 
lution to dryness; re-dissolve the residue, using 2 cc. of hydrochloric acid 
and 50 cc. of water. 

5. Precipitating basic zinc carbonate. —Cautiously add sodium carbon¬ 
ate (15 cc. of a 10% solution) to the hot, moderately dilute solution, until 
the liquid is distinctly alkaline; then bring the mixture to boiling. 

Allow the precipitate to subside, decant on a filter, and wash with hot 
water. 

6. Changing zinc carbonate to zinc oxide. —Dry the precipitate. Sep¬ 
arate it as far as possible from the filter paper. Burn the paper in a por¬ 
celain crucible. To the ashes add the zinc carbonate; ignite, and weigh. 
The substance weighed is zinc oxide, ZnO. (It contains 80.34% of the metal.) 

Remarks 

In certain cases, manganese, as Mn^Og, may be present with the zinc 
oxide. 

In such cases, dissolve the contents of the crucible in dilute hydrochloric 
acid. Boil with excess of sodium hydroxide. The oxide of manganese should 
be precipitated, ignited, and weighed as MngO^. Its weight, multiplied by 
1.035, must be subtracted from the weight of the mixture with oxide of zinc 
previously obtained. 


C*—Volumetric Method for Zinc* 

Ferrocyanide Method with Uranium Indicator 
Cairns, 185, 188. Hiorns, 321, 440. 

Crookes’ Select Methods,145E. Ricketts & Miller, 153. 

Outline of the Process 

Titrate the zinc in a hydrochloric acid solution with standard potassium 
ferrocyanide solution, using uranium acetate (in concentrated solution) in 
spots on a porcelain plate, as an indicator. 






ZINC 


5 


Supplies 

1. Uranium acetate, a few cc. of a saturated solution. 

2. Pure zinc, 5 g. dissolved in 33 cc. of hydrochloric acid and diluted 
to 500 cc. (or some other convenient weight of zinc, dissolved in its proper 
amount of water). 1 cc. corresponds to 10 mg. of metallic zinc. 

3. Potassium ferrocyanide, 21.6 g. dissolved in 500 cc. of water. 1 cc. 
correspond to 0.010 g. of metallic zinc. 

Standardize this solution by the zinc solution (No. 2 above) as follows: 

Draw 50 cc. of the zinc solution; to it add 2 cc. of concentrated hydro¬ 
chloric acid and about 150 cc. of water. 

Heat the mixture to incipient boiling and keep the solution hot. 

Into the warm solution, slowly add the ferrocyanide solution from a 
burette, spotting from time to time upon drops of uranium acetate upon the 
porcelain plate. 

From the number of cc. required, compute the exact value of each cc. 
of the ferroc 3 "anide solution, in terms of metallic zinc. 

The Process 

1. Drawing the sample .—Draw from the stock solution 100 cc. (It 
represents 1 g. of ore.) Heat to boiling. 

2. Removing iron .—To the solution add about 15 cc. of concentrated 
ammonia to precipitate iron, etc., and to dissolve zinc. 

Filter, using the pump. 

Re-dissolve the precipitate of ferric and aluminium oxides in 5 cc. of con¬ 
centrated hydrochloric acid; re-precipitate with 6 cc. of concentrated 
ammonia and refilter (to secure zinc which at first went down with 
ferric oxide, etc.); add this filtrate to the foregoing filtrate. Make the solution 
to the volume of 200 cc. 

3. Titrating .—Place the prepared solution in a casserole. To it add 
15 cc. of hydrochloric acid. Heat to incipient boiling. Keep the solution 
hot. Titrate carefully and rather slowl}^ with the standard ferrocyanide so¬ 
lution, spotting from time to time upon uranium solution. (As described, 
each cc. of ferrocj^anide solution represents about 1% of metallic zinc.) 

Remarks 

Copper and iron are objectionable. 

Potassium chloride appears to be unobjectionable. 

Zinc sulphate gives only fairly good results. 

Excess of hydrochloric acid seems objectionable in titration, but distinct 
acid reaction with hydrochloric acid is desirable. 

Zinc fluoride is insoluble in ammonia; this seems to forbid immediate 
use of hydrofluoric acid on the sample. 

Silica does not appear to carry down zinc. 

Iron appears to carry down abouty%% of zinc in the first precipitation— 
this is recovered in the second precipitation. 




LEAD 

Principal Ores of Lead 


Galena, (Dana, 48), 
Lead Sulphide, 
PbS. 

S 13.4 

PI) 86.0 


100.00 

Anglesite, (Dana, 907), 
Lead sulphate, 

PbSO^ 

SO 3 26.4 

PbO 73.6 (Pb, 68.31) 


Cerussite, (Dana, 286), 
Lead carbonate, 
PbCOg. 

CO 2 16.5 

PbO 83.5 (Pb, 77.50) 


100.00 

Pyromorphite, (Dana, 770), 
Lead phosphate 
3Pb3P208.PbCl2 
P 2 O 5 15.7 

PbO 82.2 (Pb, 76.29) 

Cl 2.6 


100.00 100.5 


Lead—Synopsis of Methods 

Gravimetric Methods. Fresenius, I: 358. 

1 . Metallic lead. 

a. Potassium cyanide method. 

Reduce by fusion with potassium cyanide. 

Wash, dry and weigh. 

Dissolve in nitric acid and subtract the weight of the residue from 
the first w'eight. 

b. By metallic zinc or metallic cadmium. 

2 . Lead oxide. 

a. By precipitation. 

Precipitate with ammonium carbonate. ^ 

Ignite and weigh as PbO. 

b. By ignition. 

Heat the compound in a weighed crucible. 

Weigh as PbO. 

3. Lead sulphate. 

a. By precipitation. 

Precipitate with dilute sulphuric acid. Wash with alcohol. 
Weigh as lead sulphate, PbS 04 . 

b. By evaporation. 

Treat the compound in a weighed dish with sulphuric acid. 

4. Lead chromate. 

Precipitate with potassium dichromate in acetic acid solution. 
Weigh as lead chromate, PbCrO^. 

5. Lead chloride. Olsen, 136. 

Precipitate by hydrochloric acid in the presence of a mixture of 
alcohol and ether. 

Weigh as lead chloride, on a balanced filter. 


6 






L E A D 


7 


Volumetric Methods 

j 

1. Potassium sulphate method. Schimpf, 270. 

Titrate with the sulphate solution until a drop of the mixture fails 
to produce a yellow color upon a paper saturated with potassium 
iodide and sodium thiosulphate. 

2 . Potassium chromate method. Beringer, 174. 

Add an excess of potassium chromate to the solution. 

Determine the excess colorimetrically. 

3. Potassium dichromate method. Sutton, 253. 

Titrate with the dichromate solution until a drop of the mixture 
fails to produce a precipitate with silver nitrate. 

Dry Methods 

1. Metallic lead. Brown; Hiorns; Beringer; Ricketts. 

Fuse in a crucible or scorifier with fluxes and reducing agents. 
Weigh as metallic lead. 


Lead—Assay of Galena 

A. —Gravimetric test, sulphate method, by precipitation. 

B. —Volumetric test, chromate method with silver indicator. 

C. —Dry process for galena, using iron rods. 

A,—Gravimetric Test—Sulphate Method—Fresenius 2 : 574 

Outline of the Process 

Oxidize lead sulphide to lead sulphate and weigh as such. (But sep¬ 
arate gangue, removing lead sulphate by dissolving it in ammonium acetate.) 

The Process 

1. Weighing and oxidizing. —Weigh 2 g. of the finely powdered galena. 
Transfer it to a No. 4 beaker provided with a glass cover. 

Add 10 cc. of fuming nitric acid. Place the beaker on a hot plate. Heat 
gently for about 30 minutes. Now remove the glass cover; evaporate to 
dryness on the same plate. To the residue add 50 cc. of water, then care¬ 
fully add 3 cc. of concentrated sulphuric acid. Boil the mixture for a few 
minutes. 

2. Filtering, drying, etc. —Filter; wash the precipitate with water acidu¬ 
lated with sulphuric acid; then wash with alcohol. 

Dry the paper and contents. Remove the precipitate to a piece of 
glazed paper. Ignite the filter paper in a porcelain crucible. To the ash, 
add 5 drops of dilute nitric acid; warm the mixture; then evaporate to 
dryness. 





8 


THE A SS A Y-B 0 0 K 


% 


Transfer the precipitate to the crucible containing the filter ash, etc. 
Add 5 drops of dilute sulphuric acid. Carefully evaporate the mixture to 
dryness. Ignite gently. 

3. Weighing. —Weigh as lead sulphate and gangue. 

4. Separating lead sulphate from gangue. —Transfer dry the contents 
of the crucible to a dry beaker. Add 25 g. of solid ammonium acetate; 
then 50 cc. of water; then 2 cc. of ammonium hydroxide; then boil the 
whole until it is judged that all of the lead sulphate is dissolved. 

5. Filtering, etc. —Finally transfer everything to a filter; wash with 
hot water, dry, ignite. 

6. Weighing. —Weigh the residue as gangue. Subtract this weight 
found from that of the original residue (gangue and lead sulphate). The 
difference is the weight of the lead sulphate. 

B.—Volumetric Test—Chromate Method 
Sutton, 253; Beringer, J73 
Outline of the Process 

Boil the galena in hydrochloric acid. From the solution, precipitate 
metallic lead by means of a zinc rod.* Strip the metallic lead from the zinc. 

Titrate the lead in an acetic acid solution with a standard solution of 
potassium dichromate, using silver nitrate as an indicator. 

Supplies 

1. Silver nitrate, saturated solution. 

2. Pure zinc rods. 

3. Potassium dichromate, at the rate of 7.117 g. per litre; 1 cc. rep¬ 
resents 0.010 g. lead. 

Compute the amount of dichromate per litre necessary to precipitate 
the lead, so that 1 cc. of dichromate will be equivalent to exactly 0.010 g. 


of lead. 




Pb (N 03)2 


K 2 Cr 207 


Pb 

206.9 X 2 = 413.8 


78.30 

2N 

28.08 

Cr2 

104.2 

20 3 

96. 

0- 

112. 


330.98 X2 = 661.96 


294.50 

2Pb (N()3)3 

-|-K2Cr20- -t- HgO = 

2PbCr04 -h 2KNO3 + 2HNO3 

413.8 : 294.5 

: ; 10 g. : X the wt. of K 2 Cr 207 . x = 

7.117g. 


Standardizing.—\Ne\gh \ the quantity (3.5585 g.) of the dichromate; 
dissolve it in water and dilute the solution to 500 cc. 

Weigh 2 g. of pure metallic lead. Place this amount in a No. 4 beaker. 
Add 50 cc. of dilute nitric acid. Warm the mixture until the lead is dissolved 
Neutralize the solution with about 50 cc. of ammonia. Add a considerable 
excess of acetic acid. Dilute the whole to a volume of 500 cc. 





LEAD 


9 


For each test take 100 cc. of this solution (equals 0.400 g. of metallic lead). 

Boil the solution; then add a measured quantity of the dichromate soi 
lution until nearl}'" all the lead has been precipitated. Boil the mixture 
again until the precipitate becomes orange-colored. 

Finish the titration by carefully adding dichromate, as needed, to the hot 
solution. Determine the end-point by bringing small portions of the solution 
in contact with drops of silver nitrate placed upon a white porcelain tile. 

Compute the exact value of 1 cc. of the dichromate solution in terms 
of metallic lead. 


The Process 

1. Weighing and dissolving .—Place 2.5 g. of the finely powdered ore 
in a No. 4 beaker, with a glass cover; add 50 cc. of dilute hydrochloric acid. 
Heat the mixture until the evolution of hydrogen sulphide almost ceases. Add 
water if necessary. 

Introduce a piece of a zinc rod. Continue the heating until all the sul¬ 
phide has been dissolved and all the lead has been precipitated in metallic 
form. 

Decant the liquid; then wash the metallic residue twice with cold water. 

Strip off the precipitated lead and clean the zinc. Cover the lead with 
25 cc. of water and about 12 cc. of nitric acid. Heat gently until all the 
metallic lead is dissolved. 

Dilute the solution to a volume of 250 cc. Use for each test 50 cc. of 
this solution (equivalent to 0.500 g. of ore). 

Neutralize the 50 cc. with ammonia (2 or 3 cc.), then add a considerable 
excess of acetic acid (about 15 cc.). 

(The solution thus prepared is such that it will, in the subsequent ti¬ 
tration, require from 30 to 43 cc. of dichromate solution, representing from 
60% to 86% of metallic lead in the ore.) 

2. Titratmg .—Boil the solution. Then add the standard solution of 
potassium dichromate as in standardizing. Determine the end-point as 
before. 

3. Computing .■—From the number of cc. used and the strength of the 
dichromate solution, determined in the preliminary standardizing, compute 
the amount of lead in the sample taken. (As described, each cc. of dichro¬ 
mate solution represents about 2 % of metallic lead.) 

C—Crucible Method for Lead in Galena 

Brown, 186, 189, 190; Hiorns (Assaying), 208; Beringer, 171 

Outline of the Process 

Mix the ore with proper fluxes. These should be reducing (as the al¬ 
kaline carbonates, carbon, etc.) and desulphurizing (iron). 

Fuse the charge. 

AVeigh the metallic lead. 









10 


THE ASS A Y-B 0 0 K 


Supplies 

Crucible. Wedgwood mortar. Sodium bicarbonate. Argols. Salt. 
Borax. 

Iron rods. Each is rod to be about f in. in diameter, 6 in. long; at one 
end a portion of 2 in. to be bent over so that the whole assumes the shape 
of an L. 

The Process 

1. Preparing the charge. —Weigh the materials : 

ore 30 g. 

sodium bicarbonate, 30 g. 

borax, 30 g. 

argol, 10 g. 

Transfer them to a wedgwood mortar. Grind the mixture with a 
pestle so as to thoroughly intermingle the substances. 

2. Charging the crucible. —Transfer the mixture to a piece of paper 
and thence to the crucible. 

Into the same mortar introduce 10 g. of common salt. With a pestle 
stir this around, thus rinsing the mortar. Transfer the salt to the crucible 
so as to form a cover on the top of the principal portion of the charge. 

Place 4 iron rods in the crucible in such a way that they reach through 
the charge to the bottom. 

3. Fusing. —Place the crucible in a moderately hot fire; cover it. 
Fuse for about 30 minutes, or until the mass is red-hot, and in quiet fusion. 

When the charge is in a liquid condition, remove the cover. Draw out 
one of the iron rods; examine it and replace it. 

Stir the molten mass thoroughly with the iron rods. Then continue 
quiet fusion for 15 minutes longer. 

Withdraw the iron rods one by one and slowly (the purpose is to allow 
any adhering globules of metallic lead to run from the rods into the melted 
charge). 

Withdraw the crucible. Tap it gently with a pair of tongs (the pur¬ 
pose is to help scattered globules of lead to sink to the bottom of the crucible). 

4. Cooling and weighing. —Cool the crucible (over night if convenient). 
Break the crucible with a hammer. Break away the slag from the button 
of lead. Hammer this button into a little cube (by this means small por¬ 
tions of slag are more effectually removed). 

Weigh the button of metallic lead, and make the necessary computation. 

Experiments on the Fusion of Lead and Lead Oxide in Presence of Silicon 

Dioxide (or Silicates) 

1. Lead oxide in a crucible.—In a small sand crucible, place about 
150 g. of lead oxide, PbO, also called litharge. Place crucible and contents 



L E A D 


11 


in a good coal fire. Continue the experiment until (first) the lead oxide is 
in fusion; (second) it makes a hole in the crucible, and runs awa}^. 

Withdraw the crucible; cool it; examine it; break it and examine it 
again. 

The oxide easily fuses; it forms an easily fusible lead silicate. Thus the 
silicions matter of the ivalls of the crucible may be dissolved by the melted lead 
oxide. 

(Ijead oxide is used, with other substances, especially soda ash—sodium 
carbonate—-and sand in making flint glass. The fusible lead silicate formed 
blends with other silicates, making an easily fusible glass. Moreover, the 
density of the lead silicate is so great that glass containing it refracts light 
strongly—thus the glass has a more brilliant appearance.) 

2. Lead in a crucible.—In a small sand crucible, place about 400 g. of 
ordinary metallic lead. Then place the crucible and contents in a good coal 
fire. Continue the heating for about 1 hour. 

Withdraw the crucible; pour the melted lead—if any remains—into 
some proper receptacle. 

Cool the crucible; examine it, breaking it if necessary for inspection. 


S 



ANTIMONY 


Principal Ore of Antimony 

Stibnite, (Dana 36), 
Antimony trisulpliide 

862^3. 

S 28.6 

Sb 71.4 


100 . 

Antimony—Synopsis of Methods 

Gravimetric Methods 

1. Antimony tetroxide. Freseniiis, 1: 398. 

a. By direct ignition. (In case of compounds containing easily 
volatilized acids.) 

b. By previous precipitation as antimonious sulphide. 

Heat the dried sulphide, in a covered crucible, with fuming nitric 
acid. 

2. Antimonious sulphide. Fresenius, 1: 395. 

Precipitate the antimony with sulphuretted hydrogen; dry and 
we’gh the product. 

Correct the result (1) by heating a weighed portion of the sulphide 
in a porcelain boat in a current of carbon dioxide or (2) by act¬ 
ing on a weight portion of the sulphide with fuming nitric acid 
—thus producing antimony tetroxide. 

Volumetric Methods 

1. Iodine method. Mohr’s method. Fresenius, 1; 400. 

Dissolve the antimonious compound in presence of tartaric acid! 
neutralize the solution with sodium bicarbonate; titrate with a 
standard solution of iodine, using starch-water as an indicator. 

Sb 203 + 41 -8 4 HNaC 03 = Sb 205 + 4NaI + 2 H 2 O + 4 CO 2 

2. Dichromate method. Kessler’s method. Sutton, 162. 

(Tartaric acid is not admissible.) 

litrate with slight excess of potassium dichromate (previously 
standardized by arsenious oxide) using standard solution of 
ferrous sulphate as an opposite balancing solution, and em¬ 
ploying potassium ferricyanide as an indicator. (Potassium 
permanganate may be substituted for dichromate as the oxidizing 
agent.) 

Compute the amount of antimony from the amount of arsenious 
oxide. 

0.005 g. AS 2 O 3 corresponds to 0.007253 g. SbgOg. 

3. Hydrogen sulphide method. Schneider’s method. Sutton, 163. 

Turn the antimony into sulphide. 


12 



AN TIMONY 


13 


Decompose this antimony sulphide (either form) by boiling in hy¬ 
drochloric acid in a special apparatus, absorbing the expelled 
gas in an excess and alkaline arsenite. 

, Titrate the excess of arsenious oxide with standard solution of 
iodine, using starch-water as indicator. 

Compute the amount of hydrogen sulphide and thence the amount 
of antimony. 3 equivalents of H 2 S correspond to 1 equivalent of Sb. 

Dry Methods 

There are certain objections to dry methods (though the latter are 
sometimes used.) The antimony after reduction is volatile at furnace heat; 
moreover, in the process of reduction other metals tend to go into the anti¬ 
mony button. (When metallic iron is used to separate sulphur from anti¬ 
mony, there is this difficulty; an alloy of iron and antimony may form.) 


Antimony—Assay of Stibnite—Gravimetfk Method 

Outline of the Process 

(a.) Treat the ore with hydrochloric acid, noting the amount of in¬ 
soluble matter. 

(b.) Precipitate the antimony as sulphide, SbaSj + nS. 

(c.) Make the proper corrections for the irregular amount of sulphur. 

Supplies 

Balanced filters, 

►Sulphuretted hydrogen water. 

The Process 

1. Weighing and dissolving. —Weigh .500 g. of the very finely pow¬ 
dered ore. Transfer it to a No. 2 beaker provided with a glass cover. Add 
6 cc. concentrated hydrochloric acid, sp. g. 1.20. 

Warm the mixture to expel sulphuretted hydrogen (ljut keep the so¬ 
lution below the boiling point, to avoid loss of volatile chlorides of antimony). 

Add 1 g. of tartaric acid, then carefully add water—5 cc. at a time— 
until the volume of the solution is about 100 cc. (It is intended to prevent 
precipitation of oxychlorides.) 

2, — Determining the insoluble 'part. —Filter the solution. (Save the 
filtrate.) 

Wash the precipitate with water containing a little hydrochloric acid 
and a little tartaric acid (to prevent precipitation of oxychloride). 

Add the washings to the rest of the filtrate. 

Dry and ignite the precipitate and weigh it as insoluble matter. 

3. Determining the antimony. —In the united filtrate and washings pre¬ 
cipitate the antimony with clean sulphuretted hydrogen gas and continue 
the process as described in Appleton’s Quantitative Analysis, pages 53 to 56. 





14 


THE ASSAY-BOOK 


Antimony—Assay of Stibnite—Volumetric Method by Iodine 

Outline of the Process 

(a.) Prepare the starch-water. 

(b.) Prepare solution of hydrosodiiim carbonate. 

(c.) Prepare the iodine solution. 

(d.) Prepare the thiosulphate solution; compare this with the iodine 
solution. 

(e.) Prepare the arsenious solution; standardize the iodine solution 
by means of this one. 

(f.) Prepare the potassium iodate solution; standardize the iodine 
solution by means of this one. 

(g.) Determine antimony in the sample of stibnite. 

Supplies 

Iodine. Sodium thiosulphate. Arsenious oxide. Sodium bicarbon¬ 
ate. Potassium iodide. Potassium iodate. Starch. 

Burettes. 


The Standard Solutions 

1. The starch-water; the indicator.—Mix 1 g. starch with 5 cc. 
cold water, add 150 cc. boiling water; allow the solution to stand; filter. 

2. Hydrosodium carbonate solution.—Dissolve 6 g. hydrosodium 
carbonate, HNaCOg, in 200 cc. of water. 

3. The iodine solution.^—Weigh 13 g. iodine and 18 g. potassium 
iodide; triturate both in a mortar with small portions of water, until all is 
dissolved. Dilute the whole to 1000 cc. 

4. The thiosulphate solution. — Weigh 25 g. sodium thiosulphate, 
dissolve in water, and dilute to 1000 cc. 

(The purpose in mind is this: To use the thiosulphate solution as an 
intermediary and balancing solution—not as a basal standard. Its num¬ 
erical relations to the iodine solution must be definitely learned; then if in 
subsequent work too much iodine has been inadvertently added when ti¬ 
trating, such excess may be counterbalanced by addition of thiosulphate, 
the latter’s relation to iodine being known.) 

5. Comparing the solutions 3 and 4.—From a burette, draw 40 cc. of 
thiosulphate solution into a casserole; add about 100 cc. of water and 5 cc. 

of starch-water. X 

From another burette, cautiously draw iodine solution until permanent 
appearance of the blue of the iodo-starch. 

Compute and record the ratio of the iodine solution to the thiosulphate 
solution. 

6. The arsenious solutions.—Weigh about 5 g. of hydro-sodium car¬ 
bonate. Dissolve in 50 cc. of water. If the salt does not completely dissolve, 
use the clear solution. 




ANTI M ON Y 


15 


Weigh two portions each .200 g. of pure arsenious acid. Treat each as 
follows: Dissolve the weighed portion, with stirring, in 10 cc. sodium hy¬ 
droxide; then dilute to 150 cc. Add hydrochloric acid until slightly in ex¬ 
cess ; then neutralize with a concentrated solution of the bicarbonate. Add 
to each, 5 cc. of the starch solution. 

7. Arsenious standardizing of the iodine solution.—Draw cautiously 
the iodine solution first into one, then into the other arsenious solution 
(taking care not to pass the end-point) until the permanent appearance of 
the blue. Record the amount of iodine solution used in each case. 

From the corrected volume of the iodine solution used, compute the 
quantity of iodine in each cc. of the iodine solution. 

(The purpose in mind is this: Since the iodine used is not perfectly 
pure, it must be tested by some substance that may be accepted as pure; 
such a substance is arsenious oxide. In this case, then, the real and true 
amount of iodine is determined by means of arsenious oxide—which latter 
is assumed to be pure.) 

8. The iodate solutions.—Weigh two portions each .150 g. of potas¬ 
sium iodate. Treat each as follows: Place the weighed portion in a beaker 
and dissolve in 50 cc. of water. Add a solution of potassium iodide contain¬ 
ing 3 g. of the salt. 

Add to the mixture 10 cc. dilute sulphuric acid. Allow the whole to 
stand for three minutes. Dilute to 150 cc. 

9. Iodate standardizing of the iodine solution.—Draw from a burette 
into each iodate (now iodine) solution a measured volume of thiosulphate 
until the color of the iodine is nearly destroyed. 

Add 5 cc. starch solution; then add more thiosulphate, cautiously, 
until the blue color disappears. Finally, add iodine solution (from the 
main original standard) until the blue color is just restored. 

(Make a blank test on the potassium iodide for potassium iodate: The 
latter is usually present in small but variable proportions in commercial 
iodide.) Weigh two portions, 3 g. each, of potassium iodide such as has been 
previously used. 

Dissolve the weighed portions; add sulphuric acid; dilute to 150 cc.; 
then add starch and thiosulphate solutions till the blue color just disappears. 

(Make the proper computations.) These are three: First, from the 
number of cc. of thiosulphate used for iodate and iodide, subtract that used 
for iodide alone; the difference is the amount of thiosulphate referable to 
the action of the iodate weighed. Second, compute the true amount of iodine 
referable to the iodate weighed, and thence the amount of iodine neutraliz- 
able by 1 cc. of thiosulphate. Third, from the true amount of iodine just 
found as referable to each cc. of thiosulphate solution, compute the amount 
of iodine in the amount of iodine solution, described in 3 above. 

(The purpose of this work with iodate is this: Since the iodine used is 
not perfectly pure, it must lie tested by some substance that may be accepted 
as pure; such a substance is potassium iodate.) 

This iodate experiment amounts to a second test, in addition to the 
arsenious test, of the amount of real iodine in the standard iodine solution. 






IG 


THE ASSAY-BOOK 


Two tests are given in order to afford more certain results and also in 
order to instruct the pupil. 

10. Computing the antimony value of the iodine solution.—The tests 
already described have led to a correct estimate of the true iodine value of 
the iodine solution. 

This must be transformed into an expression of the antimony value. 

41 + + dHNaCOg = Sb^O^ + 4 Nal + 2R^O + 400^ 

Then Value in I Value in Sb 

4 X 126.97 2 X 120.2 of 1 cc. iodine sol. of 1 cc. iodine sol. 

487.88 : 240.4 :: a: : y 

The Process 

1. Y eighing and dissolving. —Weigh .400 g. of finely powdered stibnite. 
Place the powder in a dry beaker, No. 2, with a cover. Add 5 cc. concen¬ 
trated hydrochloric acid, sp. g. 1.20. Warm the mixture gently; for, first, 
all sulphuretted hydrogen must be expelled; second, the liquid must not be 
boiled lest volatile antimonious chloride be driven off. 

When the residue is white, add 1 g. of solid tartaric acid. Cautiously 
add, in quantities of 5 cc. at a time, 120 cc. of water. 

If white oxychloride is produced, discard all the material, and start 
the experiment again. 

If at any time red sulphide of antimony appears (showing incomplete 
(expulsion of sulphuretted hydrogen),warm the solution (before adding 
more water) until it becomes colorless; so continue until, after addition of 
all the water, a clear and colorless solution has been obtained. Cool this 
solution. 

2. Neutralizing.—CnreluWy pour the solution, little by little, into the 
sodium carbonate solution (avoiding loss by violent effervescence). 

3. Titrating. —To the solution add 2 cc. of starch-water. Titrate with 
the standard iodine solution until the blue color appears—avoiding much 
excess of iodine. 

(If much excess of iodine is accidentally added, thiosulphate solution 
may be run back, and afterwards allowed for.) 

4. Compiding.—From the value in antimony of the iodine solution 
compute the quantity of antimony in the ore examined, and thence the 

. percent. 




TIN 


Principal ores of Tin 


Cassiterite, (Dana, 234), 


Stannite, (Dana, 83), 

Tin dioxide. 


Tin sulphide. 

SnOa- 


Cu^S.P^eS.SnS^. 


S 

29.9 


Sn 

27.5 

21.4 

Cu 

29.5 

78.G 

Fe 

13.1 

100.00 


100.00 


Tin—Synopsis of Methods 

Gravimetric Methods 

I, Stannic oxide, Freseniiis, 1; 405. 

a. By treatment with nitric acid. 

Oxidize the tin with concentrated pure nitric acid. 

Dry, ignite, and weigh as stannic oxide. 

b. By precipitation as stannic (or metastannic) acid. 

Oxidize all the solution of tin to the stannic form. 

Precipitate with ammonium nitrate or sodium sulphate. 

Dry, ignite, and weigh as stannic oxide. 

SnCli -f dNa^SOi + SH^O ^H^SnOg -f 4NaCl + 4IINaSO.i 
SnCl^ + 4Nn4N03 + 3H,0 = H^SnOg + 4X14^01 + 4HNO3 

c. By precipitation as sulphides, -ous or -ic. 

Precipitate in moderately acid .solution with hydrogen sulphide. 
Pdlter and wash the precipitate with the aid of a salt solution. 

Dry and heat, in the presence of air, to drive off the sulphur. 
Penally ignite strongly with ammonium carbonate until the weight 
is constant. 


Volumetric Methods 

1. By iodine in alkaline solution. Lenssen’s method, P'roseiiius, 1: 408, 

Dissolve the substance in hydrochloric acid. 

Deduce to the stannous form. • 

Add sodium potassium tartrate and then sodium bicarbonate in 
excess. Add starch solution. Titrate with a standard iodine 
solution. 

2. Ferric chloride method. I^owenthaPs method, Sutton, 373, 

Reduce the tin to the stannous form. 

Treat with a concentrated solution of ferric chloride. 

Titrate the reduced ferrous chloride with a standard solution 
of potassium permanganate. 

SnCl2 + Fe.Cle = SnCl^ + 2FeCl2 


17 




18 


THE ASS AY-BOOK 


Dry Methods 

1 . Cornish method. Beringer, 232. 

Reduce the oxide with carbon (anthracite). 

Weigh the metallic tin. 

2. Cyanide method. 

Dissolve impurities in the ore with Iwdrochloric acid. 

Oxidize with nitric acid. 

Reduce the oxide by fusion with carbon and potassium cyanide. 
Weigh the metallic button of tin. 

SnOa + 2KCN = Sn + 2KCNO. 


Volumetric Method for Tin—Dichromate Method 

Outline of the Process 

Treat the ore with acid to remove iron and reducible substances. 

Heat the residue in a current of hydrogen or other reducing gas to pro¬ 
duce metallic tin. 

Dissolve the metallic tin in hydrochloric acid. 

Titrate with standard potassium dichromate. 

Supplies 

1. Pure metallic tin. 

2 . Glass combustion tube with porcelain boat. 

3. Hydrogen generator. 

4. Starch-Iodide mixture. Place in a casserole. 

powdered starch, 1 g. 

potassium iodide, 3 g. 

water, 100 cc. 

Boil the mixture with constant stirring, then allow the whole to cool 
before use. 

5. Standard solution of potassium dichromate.—Dissolve 4.125 g. 
of the pure dry salt in about 50 cc. of hot water. Dilute this solution to 
the volume of 500 cc. 1 cc. is intended to represent 10 mg. of metallic tin. 

Standardizing .—Weigh 5 g. of pure metallic tin. Place it in a small 
platinum dish. Place the latter, with its contents, in a casserole. Cover 
the tin with hydrochloric acid (150 cc. of concentrated acid with 50 cc. of 
water). Cover the casserole with a large watch glass and boil the acid. (It 
may require three or four hours to dissolve all the tin.) Add more hydro¬ 
chloric acid if necessary. 

When the tin is dissolved, dilute its solution to 500 cc. The solution 
contains stannous chloride, SnCl 2 . Each cc. contains the equivalent of 
10 mg. of metallic tin. 

(In some cases, it is better to dissolve only a part of the 5 g. of tin. In 
this case, proceed as follows: Remove the platinum dish with its undissolved 
tin. Wash the latter. Dry dish and contents. Weigh the undissolved 
tin; subtract this weight from 5 g. Dilute the solution to a volume such 
that 1 cc. of it will represent 10 mg. of metallic tin.) 





TIN 


19 


Into a casserole draw 50 cc. of the tin solution. To it, add 5 cc. of 
pure concentrated hydrochloric acid and 5 cc. of the starch-iodide mixture. 
Titrate this stannous solution with the dichromate solution. 

From the results of titration, compute the exact value of potassium 
dichromate solution in terms of metallic tin. 1 cc. of the dichromate so. 
lution is intended to represent 10 mg. of metallic tin. 

The Process 

1. Weighing and washing. —Weigh 1 g. of the finely powdered ore- 
Transfer it to a No. 4 beaker. 

To the ore add 30 cc. of hydrochloric acid and 10 cc. of nitric acid. 
Warm the mixture; later boil it; finally remove the cover and evaporate 
the whole to dryness. 

Cool the residue; add 50 cc. of water, and 10 cc. of hydrochloric acid; 
warm the mixture for 30 minutes. 

Filter the mixture. Discard the filtrate which may contain iron, cop¬ 
per, and certain other substances. Dry and ignite the residue. 

2. Reducing stannic oxide. — Transfer the residue, which contains 
stannic oxide, Sn() 2 , to a porcelain Ijoat. Ignite the boat and contents 
gently; then weigh the whole. Next place the whole in a glass combustion 
tube. Through the tube conduct a current of hydrogen gas. Heat the 
boat, etc., to a low red heat for one hour. Continue the current of gas while 
the boat is cooling. 

3. Tried Weighing. —Weigh the boat and contents. The loss of weight 
may be considered as principally oxygen from the stannic oxide. From 
the amount of this loss compute the amount of tin it represents. Of course 
the result is only approximate. 

(It is well to heat the material in hydrogen a second time, again noting 
the loss of weight.) 

4. Dissolving the tin. —Transfer the contents of the boat to an erlen- 
meyer flask provided with a funnel. Add 10 cc. of water and 10 cc of hydro¬ 
chloric acid. Boil for at least an hour in order to dissolve the metallic tin. 

5. Titration. —To the mixture in the erlenmeyer, add 5 cc. of h 3 ’dro- 
chloric acid and 5 cc. of the starch-iodide mixture. Then titrate with po¬ 
tassium dichromate. 

(As described, each cc. of the potassium dichromate represents 1 % of 
metallic tin.) 

Note 

The action of potassium dichromate upon stannous chloride, in acid 
solution, is expressed as follows: 

3 SnCl2 + + 14HC1 = 3SnCl^ -f 2KC1 -f Cr^Cle + 

mol. wt. mol. wt. metallic wt. of 

3 Sn K2Cr20Y Sn K 2 Cr 207 

357 : 294.5 :: 10 g. : = 8.249 g. 

(This amount, 8.249 g. of K 2 Fr 207 , dissolved to the volume of 1 litre, 
represents 10 g. of metallic tin; and 1 cc. represents 10 mg. of metallic tin.) 



20 


THE A SS AY -BOO K 


Dry Method for Tm—Cyanide Method. Beringfer, 234 

Outline of the Process 

Dissolve the impurities from the ore with hydrochloric acid. 

Reduce the stannic oxide by fusion with potassium cyanide and char¬ 
coal. 


Supplies 

Hydrochloric acid. Nitric acid. Potassium cyanide. Charcoal. 

The Process 

1. Dissolving impurities .—Weigh 10 g. of the finely powdered ore. 
Transfer to a No. 4 beaker. 

Add to the ore 50 cc. of dilute hydrochloric acid and 3 cc. of concen¬ 
trated nitric acid. Digest the mixture on a hot plate for thirty minutes or 
until the iron and other oxides are entirel.y dissolved. 

Filter off the solution; wash the remaining mass, Sn02, with hot water, 
and dry thoroughly. 

2. Reducing the stannic oxide .—Place the filter paper and its contents 
in a crucible and ignite to burn off the paper and to drive off the last traces 
of moisture. 

Mix the residue from above with 10 g. of finely powdered potassium cy¬ 
anide and 2.5 g. of powdered charcoal; place the mixture in a crucible; 
cover the charge with another 10 g. of potassium cyanide. 

Fuse at a dull red heat for half an hour. Remove the crucible and 
allow it to cool. Break the crucible; detach and clean the button. 

Weigh the button of metallic tin. 





COPPER 

Classification of Ores of Copper 

a. Those containing the iincombined metal. 

Native copper. 

b. Those practically free from snlphnr (chiefly oxides and carbonates). 
Cuprite, CiigO, 

Melaconite, Cut), 

Malachite, CuCO.j.CuOgHg, 

Aziirite, 2 CuC ()3 .CiiO 2 H ^. 

c. Non-arsenical sulphur ores, generally containing iron. 

Copper glance, Ci^S, < 

Chalcopyrite, CuaS. Fe 2 S 3 , 

Bornite (Erubescite), 3 Cu 2 S.Fe 2 S 3 . 

d. Arsenical sulphur ores, often very complex. 

Tetrahedrite (gray ore, fahl ore). 

4 (Cu 2 S.Fe 8 .ZnS.AgS.PbS) (Sb 2 S 3 .As 2 S 3 .) 


Principal Ores of Copper 


Cuprite, (Dana, 200), 

Melaconite, (Dana, 209), 

Cuprous oxide, 


Cupric oxide. 

CU 2 O. 


CuO. 

0 11.2 

0 

20.2 

Cu 88.8 

Cu 

79.8 

100. 


100. 

Azurite, (Dana, 295), 

Malachite, (Dana, 294), 

Basic cupric carbonate, 


Basic cupric carbonate. 

2CUCO3.CU (0H)2. 


CUCO3.CU (011)2. 

CO 2 25.0 

CO 2 

19.9 

CuO 09.2 (Cu, 55.3) 

CuO 

71.9 (Cu, 57.4) 

II2O 5.2 


8.2 

100. 


100. 

Chalcocite, copper glance. 

(Dana 55) 

Chalcopyrite, (Dana, 80), 

Cuprous sulphide, 


Sulphide of copper and iron. 

CU2S. 


Cu2S.Fe2S3. 


S 

35. 

S 20.2 

Cu 

34.5 

Cu 79.8 

Fe 

30.5 

100. 


100. 

Bornite, (Dana, 77), 


Tetrahedrite, (Dana, 137), 

A sulphide of copper and iron. 

A complex antimonial sulphide. 

3Cu2S.Fe2S3. 


4Cu2S.Sb2S3. 

S 28.1 

S 

23.1 

Cu 55.5 

Sb 

24.8 

Fe 10.4 

Cu 

52.1 

100. 

21 

100. 










22 


THE A .S^/l Y-BOOK 


Copper—Synopsis of Methods 

Gravimetric Methods 

1. Cupric oxide. Frcsenius, 1: 371. 

M. lly direct precipitation. 

Precipitate from a boilinj^ solution witli sodium hydroxide or po¬ 
tassium hydroxide. 

Ignite and weigh as CuO. 

1). By ignition. 

Heat the compound in a weighed crucible. 

Weigh as CuO. 

2. Metallic copper. 

a. By metallic zinc or metallic iron. 

b. By electricity. 

3. Cuprous sidphide. 

a. By precipitation as cupric sulphide. 

Precipitate from a boiling solution with hydrogen sulphide. 

Dry and transfer to a weighed crucible. 

Deduce by strongly igniting in the presence of sulphur and a cur¬ 
rent of hydrogen or of illuminating gas. 

Weigh as cuprous sulphide, CugS. ) 

b. By precipitation as cuprous sulphocyanate. 

Precipitate by potassium sulphocyanate, KSCN, in the presence 
of sulphurous acid. 

Deduce the dried cuprous sulphocyanate with sulphur and hy¬ 
drogen as in a. 

Weigh as cuprous sulphide, Cu^S. 

Volumetric Methods 

1. Potassium iodide method, DeHaen’s method: Sutton, 201; Schimpf, 204. 

Titrate with potassium iodide in excess. 

Determine the free iodine with sodium thiosulphate. 

2CUSO4 -b 4KI = 2K^SO^ -b Cu,I, + I2 
1 at. wt. of iodine = 1 at. wt. copper. 

2. Potassium cyanide method, Parkes’ method; Sutton, 204. 

Make the copper solution ammoniacal. 

Titrate with the cyanide until the blue color is discharged. 

3. Sodium sulphide in alkaline solution; Sutton, 206. 

Titrate with the sulphide until the blue color is discharged. 

4. Steinbeck’s method (especially for copper ores). 

Standardize the cyanide solution. 

Separate the copper from the ore by aqua regia. 

Separate the copper from the other metals by precipitation upon 
a zinc-platinum couple. 

Dissolve the copper in nitric acid and neutralize with ammonia. 
Titrate with the standard cyanide, as in Parkes’ method. 





COPPER 


23 


Dry Methods 

1. Cornish method; Beringer, 136. 

Concentrate the ore to a regulus, 

, Separate the sulphur by calcining. 
Reduce the copper by fusion. 
Refine the copper obtained. 


Copper—Assay of Chalcopyrite 

A. —The stock solution. 

B. —Gravimetric electrolytic test. 

C. —Gravimetric test producing cuprous sulphide. 

D. —Gravimetric test producing cupric oxide. 

E. —Volumetric test by potassium cyanide. 

F. —Dry process. 


A.—The Stock Solution 

The intention is to produce, from 10 g. of the ore, one stock solution 
of the volume of 1 litre. Next it is intended to use separate parts of this 
iolution for separate^ gravimetric and volumetric tests. (But of course 
ndependent portions of ore must be used for the dry assays.) 

Supplies 

Nitric acid, 10 cc. (sp. g. 1.20), diluted with water to 50 cc. 

Hydrochloric acid, 50 cc. (sp. g. I.IG). 

Outline of the Process 

Treat the powdered ore with aqua regia. Evaporate this solution to 
dryness. 

Add hydrochloric acid and boil the mixture. 

Filter the solution; burn the insoluble part to remove sulphur, etc. 

Redissolve the ashes from this process, adding their solution to that 
previously obtained. 


The Process 

1. Treating with aqua regia .—Weigh 10 g. of the finely powdered ore. 
Transfer it to a No. 4 beaker. Add 50 cc. of hydrochloric acid. Boil the 
mixture about 30 minutes on a hot plate. Then add carefully 50 cc. of nitric 
acid (mentioned above.) Evaporate the mixture to dryness on the hot 
plate. Cool the beaker and contents. 

2. Treating with hydrochloric acid .—To the cool mixture add 50 cc. of 
water and 10 cc. of concentrated hydrochloric acid. Boil the mixture again 
on the hot plate. 

Filter the solution; with water wash the residue on the paper. 




24 


r II E A SSA Y-IWOK 


0 

3. Second treating of the insoluble part. —Dry the insoluble residue, and 
when dry, burn the paper and contents in a porcelain crucible. Return the 
ashes to the same beaker used for dissolving before. 

To the ashes in the beaker, add 2 cc. of hydrochloric acid and Icc. of 
nitric acid, both concentrated. Place the beaker and its contents on a hot 
plate and evaporate the material to dryness. 

4 o the cool and dry residue add 25 cc. of water and 2 cc. of hydrochloric 
acid. Boil the mixture again. 

Filter the solution, washing the precipitate. 

(It is always well, before final dilution to 1000 cc. as described below, 
to treat the insoluble matter 5mt again with acids and to test a portion 
of the new filtrate for copper. The purpose is to make sure that the pre¬ 
ceding operations have extracted all the copper from the ore.) 

4 he filtrate is now supposed to have taken out the last traces of copper. 
Return this filtrate to the preceding copper solution. 

4. Completing the stock solution. —Make this solution to the volume of 
1000 cc.; 100 cc. of it represents 1 g. of the original ore. 


B.—Gravimetric Method for Copper—Electrolytic Method 

Brown, 211 j Olsen, 193, 206 

Remark. The electrolytic method is to be recommended in most 
cases. It is especially useful in industrial laboratories, in which large num¬ 
bers of assays must be made. 

But this method faces difficulties in presence of arsenic, antimony 
or Ihsmuth. 

Outline of the Process 

Dissolve the copper from the ore. 

krom the solution, precipitate the copper electrically upon a platinum 
electrode. 

Supplies 

Two selected small beakers, appropriate for receiving the platinum 
electrodes. 

The galvanic battery (or other source of electric current), with volt¬ 
meter and ammeter. 

The platinum electrodes. (Before use, clean, the electrodes with sand- 
soap and water then with dilute nitric acid. Finally rinse the negative 
electrode—the cathode—with ethyl alcohol. Before the alcohol dries, set 
it on fire. Cool the cathode in a desiccator and then weigh it. 

The Process 

1. Preparing the solutions.—Vrepare two solutions, just alike, as follows: 
From the stock solution measure into a small evaporating dish 50 cc.; it 
represents .500 g. of ore. 4"o it add 5 cc. of concentrated sulphuric acid; 





COPPER 


25 


then evaporate the solution nearly to dryness (or until white' fumes of 
sulphuric acid beo:in to escape.) 

To the residue add 50 cc. of water; decant the clear solution into one 
of the selected small beakers. To any undissolved residue add aqua regia^ 
drop by drop, heating meanwhile. (The -purpose is to dissolve all of the 
copper, but to introduce as little aqua regia as possible.) 

.\dd the aqua-regia solution to the preceding sulphuric acid solution. 
To the whole add about 1 g. of ammonium nitrate. Now the small beaker 
has the solution ready for deposition of metallic copper. 

2. Depositing the metallic copper. —In the solution place the proper 
platinum pieces—anode (the spiral); cathode (the cylinder). Connect with 
the battery so that the cathode is attached to the terminal wire from the 
zinc battery-plate: i.e., with the negative pole. 

Allow the current (about 2—5 volts, and about .2—.3 amperes) to act 
until apparently all the copper is out of the solution—perhaps 12 to 24 hours, 
or, in some cases, more time may be required. 

Kemove the electrodes and save the solution. 

3. Washing and weighing the coated cathode. —Rinse the cathode with 
water and then with ethyl alcohol. Set the alcohol on fire. (The heat af¬ 
forded dries the coated cathode, \mder reducing conditions.) 

Place the cathode in a desiccator to cool; weigh it. 

(Subsequently place the cathode in a small beaker containing moder¬ 
ately concentrated nitric acid; the copper should dissolve. Wash the cathode 
with water.) 

4. Testing the solution. —To the solution, after deposition of the copper, 
add about 5 cc. of ammonia (or enough to render the solution alkaline); if 
the solution does not become blue, practically all copper has been removed. 

(In some industrial works, any such solution becoming blue is com¬ 
pared with several already prepared blue solutions, each one of which contains a 
different, small, known amount of metallic copper. If the l:)lue solution, 
afforded l}y the assay, matches color with one of the standards, then the 
numerical amount previously obtained is enlarged by adding an amount cor¬ 
responding to that represented by the matched blue standard.) 


Notes on the Electrolytic Precipitation of Copper 

Introduction 

An alternating current cannot be used. One of its impulses would 
throw copper ions upon the platinum electrode; at the next reversed 
impulse the ions would be thrown back into the solution. 

Idle direct current separates the copper continuously. 

Dynamo Currents and Galvanic Currents 

Within certain limits either current may be used. 

The dgnanio direct current. —If this current is of low voltage, say 50 volts 
or lower, it can be conveniently used. Practically all that is necessary is to 




2G 


THE ASSAY-BOOK 


introduce resistance in the form of a single lamp in the circuit. The lamp 
usually reduces the voltage sufficiently for our purposes. 

At the present time many laboratories are supplied with a direct cur¬ 
rent from a dynamo. But this current is often of 250 volts. So high a 
pressure is entirely unnecessary—indeed, unless carefully employed, it may 
work injury to the experimenter. With proper care, such a current may 
be passed through a single lamp as a resistance and then into the solution 
to be tested; notwithstanding the resistance, there is still sufficient pres¬ 
sure for the purpose. Observe, however, that these high pressure currents 
are always “grounded”: If, then, the experimenter connects himself with 
the ground by means of a metallic water-faucet, he is in danger of gettmg 
a 250 volt charge through his person. 

A second difficulty may be mentioned: When the volt-meter is attached 
to his appliances, the experimenter may, in a moment of forgetfulness, re¬ 
move his solution while the current is flowing; then the high-pressure cur¬ 
rent flows through the volt-meter, whose fine wires may be melted or even 
vaporized—with a destruction of the contrivance or under some cicum- 
stances with its explosion. 

The galvanic current .—This current is generated by a galvanic battery. 
The general principle of the battery is such that it demands a suitable ves¬ 
sel and three substances. One is usually a solution of an acid or salt; one 
is a comparatively inert metal (or maybe carbon); one is a metal (often 
zinc) which is acted upon by the solution with greater chemical energv than 
is the other metal (or carbon). Evidently this general description allows 
use of a great variety of substances : it is thus that the great variety of bat¬ 
teries is accounted for. 

It must be noted that the three substances in a battery must absolutely 
be conductors of the electricity. Sometimes one or more of them being a poor 
conductor gives rise to what is called internal resistance in the cell. 

Several cells may be used together. They may then be arranged in 
series or in multiples (or in a combination). 

The series arrangement increases the voltage (provided the internal 
resistance of each cell is not too great). 

The multiple arrangement gives no more voltage than a single cell; it 
practically produces a larger cell and thus increases the amperage. 

The gravity battery .—The simplest gravity battery consists of a jar con¬ 
taining (first) two water solutions, (second) a piece of copper with its con¬ 
necting wire (third) a piece of zinc with its connecting wire. 

The lower water solution is a saturated solution of copper sulphate, 
and in it is placed the piece of copper, its wire leading upward and out. 

The upper solution is a water solution of zinc sulphate, and in it is sus¬ 
pended the piece of zinc with its conducting wire leading upward and out. 

As the upper solution is lighter in weight than the lower solution, it 
sometimes is allowed merely to float upon the other. In some cases, how¬ 
ever, it is more convenient to keep the upper solution, with its piece of zinc, 
in a porous cup, the latter and its contents hanging in the upper part of the 
copper sulphate solution. 



COPPER 


27 


In galvanic batteries, generally, there is but little chemical action when 
the leading wires are disconnected. The moment the leading wires are 
brought into terminal contact, chemical action commences in the cell and an 
electric current flows through the wires. 

In the gravity battery described, when the terminal wires are connected, 
the action is approximatelij as follows: 

The SO4 ions of the copper sulphate attack the metallic zinc, form zinc 
sulphate, and at the same time they liberate their electric charges, the latter 
flowing through the wire. 

The Cu ions of the copper sulphate deposit upon the metallic copper 
and liberate their electric charges. 

It is these electric charges repeatedly delivered which constitute the 
current. 

A Few Electrical Units 

The volt .—The volt is the unit of electro-motive force; it is the force 
needed to drive one ampere through the resistance of one ohm. 

It is approximately the electro-motive force of one galvanic cell. 

Voltage may be compared to the pressure of water in a pipe. 

The ampere .—The ampere is the unit of electric current (in quantity); it 
is the quantity of current that may be driven through the resistance of one 
ohm by the pressure of one volt. 

An ampere is the quantity of current that deposits 

about 18.302,400 grains (1.186 g.) of copper per hour 
about .305,040 grains (.020 g.) of copper per minute 
about .005,084 grains (.000,329 g.) of copper per second. 

Amperage may be compared to the quantity of water passing through 
a ihpe. 

The coulomb .—The coulomb is the unit of electric quantity (in time). 
It represents one ampere-second. 

It has been found that 96,540 coulombs of electricity sent through a 
circuit will decompose, under proper conditions, the graynmes-equivalent of 
any conducting compound: 

96,540 coulombs will decompose 58.50 g. NaCl 

96,540 coulombs will decompose 169.97 g. AgNO^ 

96,540 coulombs will decompose 79.83 g. Cu SO4 

Hence under proper conditions,one ampere in one second (one coulomb) 
will liberate as follows: 

1 coulomb will liberate .000 367 g. Cl 
1 coulomb will liberate .000 117 g. Ag 
1 coulomb will liberate .000 329 g. Cu 

These numbers are obtained by dividing the number of grammes repre¬ 
senting the proper molecular equivalent by 96,540. 

The coulomb may be compared to the amount of water delivered from 
some contrivance in a unit of time. 

The ohm .—The ohm is the unit of electrical resistance; it is the resist¬ 
ance of about 106.3 c.m. of mercury (1 m. m. square section) at 0°C. 

(It is approximately the resistance offered to a current of electricity by 
a wire of pure silver or copper whose diameter is 1 millimetre and whose 



28 


THE A SSAY-BOOK 


length is 48.61 metres, the test being made at 18.3 degrees C. 65 
degrees F. 

It may be roughly compared to the friction experienced by water in 
flowing through a pipe.) 

Resistance of Copper Wire at 20^^ C 


No. 

1- 

-8,083 

ft. = 1 

ohm. 

No. 

6— 

-2,535 

ft. = 1 

ohm. 

No. 

12- 

- 800 

ft. = 1 

ohm. 

No. 

16- 

- 249 

ft. = 1 

ohm. 

No. 

27- 

- 78 

ft. = 1 

ohm. 


C.—Gravimetfic Method for Copper—Cuprous Sulphide 

Freseniust 1: 375 
Outline of the Process 

Precipitate the copper as cupric sulphide with hydrogen sulphide. 

Reduce the cupric sulphide by ignition with sulphur and sulphuretted 
hydrogen. 

Weigh as cuprous sulphide, CuaS. 

Supplies 

Hydrogen sulphide generator. Rose’s crucible. 

The Process 

1. The (solution. —Draw 100 cc. of the stock solution of copper. Evapo¬ 
rate to dryness upon a water bath. 

Dissolve the residue in a few cc. of dilute hydrochloric acid; again evapo¬ 
rate to dryness to remove the last traces of nitric acid. 

Finally dissolve the residue in 10 cc. of dilute hydrochloric acid, add¬ 
ing a drop or two of nitric acid, if necessary, to clear the solution. Dilute the 
whole to 200 cc.; heat to boiling. 

2. Precipitating the copper .—Into the boiling solution pass the hydro¬ 
gen sulphide gas for half an hour, keeping the mixture warm upon a hot 
plate. 

Allow the precipitate to subside; add 25 cc. of strong hydrogen sulphide 
V ater to make sure of thorough precipitation. (If no color or precipitate 
appears, proceed with the filtering, etc. but if the solution is at all colored, 
again pass hydrogen sulphide through the mixture for 30 minutes and test as 
before.) 

When thoroughly precipitated, filter the cupric sulphide quickly; wash 
with hydrogen sulphide water continuously; then dry the filter paper etc. 
quickly. ’ 

3. Reducing the cupric Transfer the dried filter, etc., to a 

weighed Rose porcelain crucible. 




COPPER 


29 


Add 1 g. of pure powdered sulphur. 

Adjust the sulphuretted hydrogen apparatus and pass the gas into the 
crucible in a moderate stream. 

Heat the crucible at first gently for 30 minutes; then raise the temperature 
to intense redness for 5 minutes. Finally allow the crucible to cool with a 
continued transmission of the gas. 

4. Weighing .—Weigh the precipitate as cuprous sulphide, CU2S. 


D.—Gravimetric Method for Copper—Oxide Method 

Outline of the Process 

a. Precipitating as sulphide. 

1. Dissolve the copper from the ore as above. 

2. Precipitate the copper as sulphide by sulphuretted hydrogen. 

b. Separating from other metals. 

1. From the copper sulphide remove sulphides of arsenic, antimony, 

and tin by the use of sodium sulphide. 

2. Dr}^ the copper sulphide left after the treatment with sodium sul¬ 

phide. Separate from the paper; burn the latter, and add the 
ashes to the mass of copper sulphide. 

3. Dissolve the sulphide in nitric acid and remove the sulphur, which 

should be pure yellow in color. 

c. Precipitating as hydroxide. 

1. Precipitate the copper as cupric hydroxide by the use ot sodium 

hydroxide. 

2. Finish the analysis as usual in this case. 


E.—Volumetric Method—Potassium Cyanide Method 
Sutton, 204, 210. Beringer, 154 
Outline of the Process 

Dissolve the copper from the ore by aqua regia. 

Separate copper from other metals by a zinc-platinum couple. 

Titrate with standard potassium cyanide in an ammoniacal solution. 

Supplies 

1. Nitric acid, 1 pt. water and 1 pt. nitric acid (1.20 sp. g.). 

2. Ammonium hydroxide, 0.90 sp. g. 

3. Pure copper. 

Dissolve 2.0 g. of pure copper (electrolytically precipitated) in 100 cc. 
of the nitric acid. Boil to expel nitrous fumes. 

Dilute the copper solution and cool. Neutralize with sodium h3'droxide, 
and when just neutral add 5 cc. of strong ammonia. Make the solution 

to 500 cc. 

4. Pure zinc. 

5. Platinum foil. 





30 


TII E .1 SS AY -BOO K 


6. Potassium cyanide, (at tlie rate of 42 g. per litre, 1 cc. corresponds 
to 0.010 g. of metallic copper). 

Standardizing. —Dissolve 21 g. of potassium cyanide in water and dilute 
the solution to 500 cc. 

Standardize this solution by means of the copper solution. 

Place in a beaker 100 cc. of the copper solution (.400 g. metallic copper), 
h rom a burette draw into the blue solution sufficient potassium cyanide so¬ 
lution to accomplish the decolorizing. 

From the number of cc. required compute the exact value of the cyanide 
solution in terms of metallic copper. 

The Process 

1. The solution. Draw 100 cc. from the stock solution, (It repre¬ 
sents one gram of ore.) 

2. Separating the copper from other metals. —Place in the beaker a rod 
of metallic zinc weighing about 50 g. and fastened to a piece of stout plati¬ 
num foil. 

Allow the precipitation ol the copper to proceed for several hours or 
over night. After the operation has proceeded for about two hours, add 5 
cc. of hydrochloric acid. (Finally test with hydrogen sulphide to make sure 
of thorough precipitation.) 

(If the beaker is placed on a steam radiator, on a warm iron plate, or 
in a warm w'ater-bath, about 2 hours suffices for complete precipitation of 
the copper. If there is a deficiency of acid present there is danger of 
precipitation of an iron compound, perhaps hydroxide.) 

Remove the excess of zinc but leave the platinum foil. Wash the cop¬ 
per repeatedly and carefully with fresh water. 

To the spongy copper in the beaker, add 20 cc. of .the special nitric acid. 
Heat moderately until the copper is dissolved; allow the solution to cool. 

3. Titrating.—Ivmi before titration, neutralize the solution wdth so¬ 
dium hydroxide and when neutral, add 1 cc. of ammonium hydroxide. Di¬ 
lute the solution to 500 cc. For each test, use 100 cc. (representing 1 g. of 
ore.) 

Titrate wdth the standard C3"anide as described above. 

(As described, each cc. ot the c^^anide solution represents about 1% of 
metallic copper.) 


F.—Dry Method—German Method—Hiorns, Mett., 20t 

Outline of the Process 

Roast the ore to oxidize the copper and to remove volatile impurities 
especially sulphur. ’ 

Reduce the oxidized copper to metallic copper. 

Refine the crude copper obtained above. 







COPPER 


31 


Supplies 

Anthracite. Argols, Glass, powdered. Sodium bicarbonate. Borax. 
Salt. Charcoal. 

The Process 

1. Roasting .—Weigh 30 g. of the finel}'' powdered ore. Place it in a 
roasting dish. 

Introduce the dish and contents into the red-hot furnace. Heat the 
ore with stirring until sulphur dioxide ceases to be evolved. 

Remove the roasted mass from the dish and mix with 15 g. of anthra¬ 
cite. Roast again for 15 minutes. 

2. Reducing .—Remove the roasted mass a second time; mix with 
10 g. of argols; replace in the crucible. 

Mix 30 g. sodium bicarbonate, 15 g. of powdered glass, and 10 g. of borax; 
place the mixture on top of the first portion of the charge. 

Cover the whole with 10 g. of salt and add a piece of charcoal. 

Place the crucible in a furnace; raise the temperature to a very bright 
red heat; then continue the heating for half an hour. 

Remove the crucible and allow it to cool. Break the crucible and free 
the metallic button from any crust of slag, etc. 

3. Refining .—Wrap the coarse copper in a piece of paper.with an 
equal weight of borax. Place the whole in a small red-hot scorifier. 

When the oxides of other metals have passed off and the copper but¬ 
ton sinks beneath the borax, remove the vessel with its contents. 

Cool the mass and detach the button from the slag. 

Weigh the metallic copper. 





IRON 

Classification of Ores of Iron 

Anhydrous oxides. 

Sesquioxides. 

Hematite, P'e.^Og, (specular, red ocherous, clay iron stone). 
Ilmenite, an iron and titanium oxide, reTi ()3 
Intermediate. 

Magnetite, iron sesquioxide and iron protoxide, I"e 0 .P''e 203 
Hydrous oxides. 

Limonite, hydrous iron sesquioxide, 2 Fe 203 . 3 H 2 (), (brown hema¬ 
tite, compact, ocherous, bog ore, brown clay iron stone). 

Sulphides. 

Pyrites, iron disulphide, FeS 2 
Carbonates. 

Siderite, iron protocarbonate, P^eC 03 (Spathic iron ore). 


Principal Ores of Iron 



Hematite, (Dana, 213), 


Ilmenite, (Dana, 217), 


iron sesquioxide. 

an iron and titanium oxide. 


P"e20g. 


FeTiOg, 



(or sometimes Fe 2 Ti 203 ). 



O 

31.6 

(I 

30 

Ti 

31.6 

Fe 

70 

Fe 

36.8 


100. 


100. 


Magnetite, (Dana, 224), 


Limonite, (Dana, 250), 

Iron sesquioxide and protoxide. 


Hydrous iron ses(]uioxide. 


Fe0.Fe203. 


2PA2O3.3H2O 



() 

25.7 

() 

27.6 

Fe 

59.8 

Fe 

72.4 

H 2 O 

14.5 


100. 


100. 


Pyrites, (Dana, 85) 


Siderite, (Dana, 276), 


Iron disulphide. 


Iron protocarbonate. 


PAS 2 , 


FeCOg. 

S 

53.4 

CD 2 

37.9 

Fe 

46.6 

FeO 

62.1 (PA, 48.2) 


100. 


100. 


Iron—Synopsis of Methods 


Remarks 

1. In some cases it is necessary to learn the total amount of iron ir¬ 
respective of its ferrous or ferric condition. 

Now steps may be taken to turn all the iron to the lerrous form and 
thereupon to determine it by processes appropriate to this condition. 

32 








IRON 


33 




Again, steps may be taken to turn all the iron to the ferric form and 
thereupon to determine it by processes appropriate to this condition. 

2. In some cases it is necessary to learn what amount of iron was 
originally present in the ferrous form, irrespective of anything else present. 

Now (supposing other reducing substances to be absent) processes 
suitable for determining only ferrous iron may be employed. 

3. In some cases it is necessary to learn what amount of iron was 
originally present in the ferric form, irrespective of anything else present. 

Now (supposing other oxidizing substances to be absent) processes 
suitable for determining only ferric iron may be employed. 

Gravimetric Methods 

(Gravimetric methods are almost entirely supplanted by volumetric 
methods or are employed in conjunction with them.) 

Ferrous Compounds. Fresenius, 1: 310 

1. Convert the ferrous compound into ferric iron and determine as such. 
(Oxidize by use of chlorine or bromine, or nitric acid or potassium 

permanganate.) 

Then treat the ferric compound as under the next title. 

2. Sulphide method. 

Precipitate as sulphide. Weigh as such. 

3. Gold method. 

Treat with gold trichloride. Weigh the reduced gold. 

Ferric Compounds, Fresenius, 1: 322 

1. Ferric oxide. 

a. By precipitation as ferric hydroxide. 

Precipitate with ammonium hydroxide. Weigh as Fe 203 . 

b. By precipitation as ferrous sulphide. 

Precipitate with ammonium sulphide. Change to chloride. 
Re-precipitate as ferric hydroxide. Weigh as Fe 2 03 . 

c. By ignition. 

Heat in a weighed crucible to constant weight. Weigh as Fe 203 

2. Ferrous sulphide. 

Precipitate as sulphide. Heat in a Rose’s crucible in a current of 
hydrogen. Weigh as FeS. 

For Either Ferrous Compounds or Ferric Compounds or Mixtures of Them 

1. Copper method. Fuchs’s method. Fresenius, 2: 499. 

Dissolve the ore in hydrochloric acid. 

Gxidize with potassium chlorate. 

Reduce by a weighed piece of pure copper. 

AVeigh the remaining copper. 

1 equivalent of Cu reduces 1 equivalent of Fe. 

Fe2Clg “h 2 Cu=2FeCl2 CU 2 CI 2 . 



34 


T II E A SS AY -BOOK 


2. Modified method. (In presence of considerable amounts of titanic acid.) 
Fresenius, 2:501. 


Volumetric Methods 

Ferrous Compounds 

1. Permanganate method. Marguerite’s method. Fresenius, 1: 312. 

Into the ferrous solution, draw standard solution of potassium per¬ 
manganate from a burette until permanent color appears. 

2. Potassium dichromate method. Penny’s method. Fresenius, 1: 319. 

Into the ferrous solution, draw standard solution of potassium di¬ 
chromate from a burette; to determine the end-point, take from 
time to time a drop of the mixture and apply it to a spot 
of solution of potassium ferricyanide on a porcelain plate. 


Ferric Compounds 

1. Stannous chloride method. Fresenius’s method. Fresenius, 1: 327. 

To the ferric solution add a measured but slight excess of standard 
solution of stannous chloride. To the solution, now colorless, add 
a little starch-water. Next add standard solution of iodine un¬ 
til the permanent blue color of starch-iodide appears. 

(But in some cases sufficiently accurate results are obtained by 
carefully noting the point at which stannous chloride produces 
decolorization—without subsequent estimation of the excess of 
stannous chloride.) 

2. Potassium iodide method. Fresenius, 1: 331. 

Reduce the ferric compound by adding slight excess of potassium 
iodide solution. Iodine is liberated; titrate the amount liberated 
with standard solution of sodium thiosulphate. 

3. Sodium thiosulphate method. Oudemans’ method. Fresenius, 1: 332. 

To the ferric solution add a small amount of solution of cupric sul¬ 
phate and also of potassium sulphocyanate—these act as indi¬ 
cators. To the solution so prepared, add a slight but known 
excess of standard solution of sodium thiosulphate. Titrate the 
excess of thiosulphate by corresponding standard solution of 
iodine. 


For Either Ferrous Compounds or Ferric Compounds or Mixtures of Them 

A. Reduce the material (by zinc, by sulphur dioxide, by sulphuretted hy¬ 
drogen, or otherwise). 

1. Employ the permanganate method, (mentioned above under fer¬ 
rous compounds). 

2. Employ the dichromate method, (mentioned above under ferrous 
compounds). 



IRON 


35 


B. Oxidize the material (by potassium chlorate, by nitric acid, by hydro¬ 
gen peroxide, or otherwise). 

1. Employ the stannous chloride method (mentioned above under 
ferric compounds). 

2. Employ the potassium iodide method (mentioned above under 
ferric compounds). 

3. Employ the sodium thiosulphate method (mentioned above under 
ferric compounds). 

Dry Methods 

Dry methods, while possible, are little used at present. Volumetric 
methods are almost exclusively employed for the determination of iron. 
(In complete analysis of iron ores, combinations of gravimetric with volu¬ 
metric tests are employed.) 

Iron—Assay of Limonite 

A. —The stock solution. 

B. —Gravimetric test for iron and aluminium, 

C. —Volumetric test by permanganate. 

U.—Volumetric test by dichromate. 

E.—Volumetric test by stannous chloride. 

A—The Stock Solution 

The intention is to produce from 5 g. of ore one stock solution of the 
volume of 500 cc. Next it is intended to use separate parts of this solution 
for separate gravimetric and volumetric tests. 

Outline of the Process 

(a) Roast the powdered ore, to destroy organic matter (if undestroyed, 
this will consume potassium permanganate, and thus count as iron). 

(b) Treat the cooled, roasted ore, first with hydrochloric acid. Then 
fuse the insoluble portion with sodium carbonate. Add the second sol¬ 
uble product, containing iron, to the soluble product previously obtained. 

(Incidentally determine the amount of matter insoluble in hydrochloric 
acid; also the amount of silicic oxide in the ore.) 

(c) Make the stock solution, from the soluble substances, to 500 cc. 

The Process 

1. Roasting the ore. —Weigh 5 g. of the finely powdered limonite. Heat 
the weighed portion to dull redness for ten minutes; do not overheat the 
ore (there is danger of diminishing the solubility of the oxides). 

2. Treating with hydrochloric acid. —Transfer it to a No. 4 beaker, 
^ provided with a suitable glass cover. Add 20 cc. concentrated hydrochloric 

acid, then 20 cc. of water. Boil the whole for at least two hours. 

Filter, with suction, using a platinum cone. 

The filtrate. Treat this first solution as described below. (3). 

The insoluble part. Treat this as described in paragraph (4). 

3. Evaporating the first solution. —While in its No. 4 beaker placed on 
a hot plate, evaporate this solution to complete dryness. Later add to this 
residue the second solution (4). 




30 


THE ASSAY BOOK 


4. Fusing the insoluble part .—Dry this material. Transfer it to a 
small platinum dish. Heat it to burn away the paper. To the residue add 
what is judged to be about four times its weight of pure sodium carbonate. 
Fuse the mixture (by this process it is proposed to turn silica into a soluble 
sodium silicate; to disintegrate the iron oxide so that later it may be dis¬ 
solved in acid). 

Place the cooled crucible with its contents in a No. 4 beaker. Add 
about 50 cc. of water. Boil the whole for some time or until the melted mass 
separates from the crucible and softens into pulp. Now carefully remove 
the platinum dish. 

Heat the mixture longer if necessary, carefully breaking lumps of 
melted material with a glass rod. 

Cover the beaker with a glass cover. While the cover is only partially 
removed, carefully add dilute pure hydrochloric acid, heating gently at 
the same time (this hydrochloric acid should dissolve all iron and other dark 
material; at the same time there should be produced silicic acid, partly 
soluble and partly insoluble). 

Transfer the whole of the material thus produced to the beaker which 
contains the first acid solution from the ore, and which is now either partly 
or wholly evaporated. (2 and 3, above). 

Continue the evaporation of the mixture, carrying it to complete dry¬ 
ness. 

Next place the beaker and its contents in the oven and heat the whole, 
maintaining the temperature at about 130° C. (The purpose is to completely 
dehydrate the silicic acid, turning it into silicic oxide, SiOa). 

5. Determining silica. To the residue in the beaker (consisting mainly 
of silica and compounds of iron), add water and an amount of hydrochloric 
acid, added gradually and in small quantities, sufficient while the solution 
is hot to dissolve everything but silica. 

Filter. 

The filtrate. Save this for the preparation of the stock solution, as 
described below (6). 

4 he insoluble part. Dry this material. Ignite it in a platinum dish. 
Weigh the residue. (This residue contains silica, but it may also contain 
insoluble mineral matters that have not j^ielded to the dissolving operation). 

Q.—The stock solution.—Dilute this solution to 500 cc. Of this solution 
100 cc. represents 1 g. of the original ore; of course 50 cc. represents .500 g. 

Gra-Vimctfic Test for Iron and Aluminium 

Outline of the Process 

Precipitate iron and aluminium together with ammonium acetate (this 
leaves manganese in solution). Filter. 

Re-dissolve the precipitated acetates, then precipitate with ammonium 
hydroxide. (Ihis affords a precipitate of ferric hydroxide and aluminic 
hydroxide.) Ignite and weigh iron and aluminium together as oxides. 









IRON 


37 


The Process 

1. Oxidizing and evaporating. — Draw 25 cc. from the stock solution 
(representing .250 g. of ore), and place this portion in a No. 4 beaker_ 
Add about 1 cc. of nitric acid (to oxidize the iron). Evaporate the 
solution on an iron plate. Dissolve the residue in about 10 cc. of water, 
to which a drop or two of hydrochloric acid are added; a complete solution 
must be produced, but the acid added must be the smallest possible amount- 

Dilute the solution slightly with water. Then nearly neutralize the 
acid by use of solution of ammonium carbonate. 

2. Adding ammonium acetate. —Dilute the solution approximately to 
the volume of 250 cc. Bring the mixture to the boiling point. 

Dissolve about 1 g. of solid ammonium acetate in about 10 cc. of water. 
Add this to the boiling solution containing iron, etc. 

Allow the bulky precipitate to subside and filter as soon as possible 
(if solutions at this stage become cold, the precipitated basic acetates tend 
to re-dissolve). 

3. Dissolving the basic acetates. —Through the moist precipitate pour 
warm dilute hydrochloric acid. (It should quickly dissolve the precipitate.) 

Wash the filter paper with boiling water. 

4. Precipitating the hydroxides. —To the filtrate add ammonium hydrox¬ 
ide. This should precipitate iron and aluminium together as hydroxide. 

5. Filtering, etc. —Filter the precipitate, wash, dry, incinerate, weigh. 
The result gives the amount of iron oxide and aluminum oxide together. 


C.—Volumetric Test by Permanganate 

Outline of the Process 

(a) Prepare a standard solution of potassium permanganate. 

(b) Prepare a standard solution of ferrous ammonium sulphate which 
shall balance the permanganate solution (or nearly so). 

(c) Standardize the permanganate solution by iron wire, using the 
Jones reductor. 

(d) Determine the iron in a specimen of limonite (using a portion of 
the stock solution already prepared). 

Supplies 

Amalgamated zinc. Potassium permanganate. Ferrous ammonium 
sulphate. Iron wire. Jones reductor. 

The Standard Solutions 

1. The permanganate solution. Dissolve 1.625 grams of potassium 
permanganate crystals in 200 cc. water with warming. Filter through as¬ 
bestos; cool; dilute to 500 cc.; then mix thoroughly. 

2. The ferrous solution. Pulverize 20 g. ferrous ammonium sulphate; 
dissolve the salt in water; add 5 cc. concentrated pure sulphuric acid; dilute 
to 500 cc. 



38 


THE ASSAY-BOOK 


3. Comparing the balanced solutions. From a burette draw 40 
cc. of the ferrous solution; add 10 cc, of dilute sulphuric acid; dilute to the 
volume of 100 cc. 

From another burette cautiously draw in the permanganate solution 
until a slight permanent pink color is established. 

Compute (first) the ratio of 1 cc. of permanganate solution to ferrous 
ammonium sulphate solution; (second) the value of 1 cc. permanganate 
solution in mg. of metallic iron, according to the foregoing experiment. 

4. Standardizing permanganate by metallic iron. Weigh two por¬ 
tions, each .250 g., of fine iron wire. Treat each as follows:— 

Place the wire in a beaker and to it add 100 cc. of water and 5 cc. of 
concentrated sulphuric acid; warm the mixture to promote solution. 

Prepare the Jones reductor for use (it is practically an air-tight tube 
containing amalgamated zinc). 

Pour the iron solution while hot through the reductor at a rate of about 
50 cc. a minute. Follow the iron solution without interruption (without 

air admission) with 175 cc. warm dilute sulphuric acid and next with 75 cc. 
of water. 

Cool the bottle containing the iron solution (now ferrous) under the 
water tap. 

To the cooled solution add 10 cc. dilute sulphuric acid and then draw 
into it, from a burette, the standard permanganate solution until a very slight 
permanent pink color is established; in case the end-point is over-stepped, 
add a measured quantity of the balanced ferrous ammonium sulphate so¬ 
lution; and, if desired, again add permanganate until again pink color is 
established. 

From the data thus obtained, compute the value of each cc. of per¬ 
manganate solution in terms of milligrams of metallic iron. 

The Process 

1. Drawing the solution.—Yrom. the stock solution draw 50 cc. (cor¬ 
responding to .500 g. of the original ore) into a casserole. Cautiously add 

5 cc. concentrated sulphuric acid; evaporate the mixture on a water-bath 
until it is nearly dry. 

Carefully complete the evaporation by heating over the lamp flame 
until the heavy white fumes of sulphur trioxide begin to appear (The 
purpose IS to expel chlorine, since it is liable to decompose permanganate 
and so give rise to an incorrect inference as to the amount of iron present) 

Cool the casserole; to the residue add 100 cc. of water; boil the mix¬ 
ture until ferric sulphate is dissolved. 

2. Beduc%7ig.~Y^ss the warm solution through the Jones reductor 

as previously described. ' 

3. Titrating. Titrate this solution with permanganate, in a manner 
similar to that already described above under standardizing permanganate 

4 Computmg--pTom the number of cc. of the permangana te so ’ 
lution demanded (knowing already its value per cc. in metallic iron) com- 

pute the amount of iron in the portion of stock solution tested, and thence 
the per cent of iron in the original ore. 






I RO N 


39 


D*—Volumetric Test by Dichromate 

Outline of the Process 

(a) Prepare a standard solution of potassium dichromate. 

(b) Prepare a standard solution of ferrous ammonium sulphate 
(which may be a mere intermediary solution with a recorded ratio to the 
dichromate solution). 

(c) Standardize the dichromate solution by iron wire, using a slight 
excess of stannous chloride as a reducing agent (and subsequently using 
mercuric chloride to neutralize the excess of stannous chloride). 

(d) Determine the iron in a specimen of limonite (using a portion of 
the stock solution already prepared). 

Supplies 

Ferrous ammonium sulphate. Stannous chloride. Mercuric chloride. 
Potassium ferricyanide. Hydrochloric acid, sp. g. 1.12. Iron wire. Por¬ 
celain tile. 


The Standard Solutions 

1. The dichromate solution.—Pulverize a little more than 2^ g. po. 
tassium dichromate. Dissolve exactly 2.500 g. in 500 cc. of water. 

2. The ferrous solution.—Pulverize 20 g. ferrous ammonium sulphate; 
dissolve in water, dilute to 500 cc. and add 5 cc. concentrated pure sulphuric 
acid. 

3. The stannous solution.—Dissolve about 5 g. pure stannous chlo¬ 
ride in pure concentrated hydrochloric acid. Dilute the solution to 500 cc. 

4. The mercuric solution.—Dissolve 25 g. mercuric chloride in 500 
cc. water; boil the mixture at first, but cool it before use. 

5. The ferricyanide solution; the indicator. Dissolve a crystal of 
potassium ferricyanide (the size of a pin-head) in 25 cc. water, 

6. Comparing the balanced solutions.—From a burette, draw 40 cc. 
of the ferrous solution, add 15 cc. hydrochloric acid, dilute to 150 cc. Next 
from another burette, cautiously draw the dichromate solution into the 
ferrous solution. 

Remove a small drop of the mixed solutions on the end of a stirring 
rod and add it to a drop of the ferricyanide on the porcelain tile. Add the 
dichromate solution to the ferrous solution until it no longer affords a 
blue color with the indicator. 

From the corrected volumes of the solutions used, compute (first) 
the value of the dichromate solution in terms of metallic iron, (second) 
compute the value of the ferrous solution in terms of the dichromate solution. 

7. Standardizing the dichromate solution by iron wire .—Weigh two 
portions each .250 g. of iron wire free from rust. Treat each as follows: 

Place each piece in a covered beaker; to it add 30 cc. of hydrochloric 
acid; warm the whole. (The solution contains chiefly ferrous chloride— 
necessarily so because hydrogen, a reducing agent, is liberated.) 







40 


THE A SS A Y-B 0 O K 


To the hot solution, add solution of stannous chloride until the iron 
solution becomes colorless; avoid having stannous chloride in an excess of 
more than one or two drops. Dilute the solution by adding 150 cc. of water; 
then cool it. Next add rapidly 30 cc. mercuric chloride solution; allow the 
solution produced to stand for three minutes. (The mercuric chloride takes 
up excess of stannous chloride, but has no influence on the stannic chloride.) 
Fe2Cl6 -h SnCla = 2FeCl2 + SnCl^ 

SnCl2 + 2HgCl2 = SnCl4 + Hg2Cl2 

Titrate with dichromate solution and determine the end-point as in 6. 

Compute the value of each cc. of the dichromate solution in terms of 
metallic iron. 


The Process 

1. Drawing the solution.~¥vom the stock solution, draw 50 cc. (cor¬ 
responding to .500 g. of the original ore); transfer this portion to a casserole. 
Heat the solution to boiling. 

2. Reducing, etc. To the hot solution, add carefully stannous chloride 
solution until the disappearance of any yellow color indicates complete re¬ 
action of ferric compounds. (Avoid an excess of more than a few drops 

of stannous solution.) 

Cool the reduced solution. Add about 30 cc. of mercuric solution. 

3. Titrating. Titrate with standard dichromate solution, spotting 
on leiricyanide drops, just as in standardizing already described. 

4 Computing.—¥mm the number of cc. of dichromate solution re¬ 
quired (knowing already its value per cc. in metallic iron), compute the 
amount of iron, and thence its per cent in the limonite. 


E—Volumetric Test for Iron—Stannous Chloride Test 

Outline of the Process 

(a) Prepare the standard solutions required. Make a tabular state¬ 
ment, showing how many milligrammes of metallic iron are represented by 
one cubic centnnetre of each of the standard solutions used. 

1 4- tested to the ferric form; then heat the so¬ 

lution to boiling. 

(c) Into the hot solution, which has a deep yellow color, draw a slight 
excess of standard stannous chloride solution from a burette. 

(d) Cool the colorless solution produced by (c),' and add a few cubic 
centimetres of starch-water. 

1 4-^^ solution of iodine, estimate the excess of tin 

solvit'ioii cIcIcIgcI. 

(f) Make the necessary computations. 


The Standard Solutions 

Prepare solutions as follows: 

1. The Ferric Soluticn.-ms is a standard solution of ammonio-ferric 
sulpliate, iron alum, (NHJ, SO, Fe, (SO,)^ 24 H, O. Dissolve 10.775 




IRON 


41 


grams of the crystallized salt in a small amount of hot water, adding a 
little hydrochloric acid if necessary; dilute the solution to the volume of 
250 cubic centimetres. One cubic centimetre of this solution contains the 
equivalent of 5 milligrams of metallic iron. 

2 . The Stannons Solution .—This is a solution of stannous chloride, 
tin crystals, (Sn Cl 2 +2 H 2 (0- Dissolve about 2.5 grams of the crys¬ 
tals in pure hydrochloric acid; dilute the solution to the volume of 500 cc. 

When tin crystals are added to water, there sometimes appears a basic 
salt of tin, which is almost completely insoluble in water and difficultly sol¬ 
uble even in dilute hydrochloric acid. The formation of this salt is pre¬ 
vented by dissolving the tin crystals at once in. boiling pure concentrated 
hydrochloric acid and afterward diluting the solution with warm water. 

It must be remembered that the tin solution is not permanent, conse¬ 
quently it must be tested afresh from time to time. 

3. The Iodine Solution .—Weigh about 500 milligrams of the puri¬ 
fied iodine; dissolve it in water by aid of a few crystals of potassium iodide; 
dilute the solution to the volume of 250 cubic centimetres. 

4. Starch-Woter .—Prepare this by softening about 3 grams of 
starch in 100 cubic centimetres of boiling water; allow the mixture to cool. 

The Value of the Iodine Solution 

Determine the relation between the tin solution and the iodine solution. 
Proceed as follows: Draw from a burette, into a clean casserole or beaker, 
10 cubic centimetres of the tin solution ; add 5 cubic centimetres of the starch 
paste; into the mixture draw iodine solution from another burette until 
the blue color produced remains permanent after stirring. Form the num¬ 
ber of cubic centimetres of iodine solution used, the value of one cubic cen¬ 
timetre of it, in terms of the tin solution, may be calculated. 

Sn CU 4- I 2 + 2 II Cl = Sn Cl^ -f 2 HI. 

The Value of the Stannous Solution 

Take 20 cubic centimetres of the standard ferric solution (prepared as 
above described); place them in a casserole with 20 cubic centimetres of 
pure concentrated hydrochloric acid, and boil; while the solution is boiling, 
run in, from a burette, the stannous chloride solution until the yellow color 
of the iron solution is wholly destroyed. 

Fe 2 (S 04)3 -f- Sn Clg + 2 HCl = 2 Fe SO 4 -f- Sn CI 4 -f- H 2 SO 4 . 

Place the casserole, with its contents, in a basin of cold water to cool. 
When cold, add 5 cubic centimetres of the starch-water, and run in the iodine 
solution until the blue color is produced. 

Knowing the value of the iodine solution in terms of the tin solution, 
the exact number of cubic centimetres of the tin solution required for 20 
cubic centimetres of the ferric solution may be calculated. 

Repeat the operation with three different portions, of 20 cubic centi¬ 
metres each, of the ferric solution. 

From the average number of cubic centimetres of the tin solution used, 
find the value of one cubic centimetre of the tin solution in terms of iron 





42 


THE A SSAY-BOOK 


solution, and from that, by calculation, its value in terms of metallic iron. 
Work out the necessary figures for filling the blanks in the following table: 

Results of Standardizing 

10 cc, Sn CI 2 solution = . . . , cc. Iodine solution. 

. . cc. Sn CI 2 solution = 1 cc. Iodine solution. 

20 cc. Iron solution = . . . cc. Sn Cl 2 solution. 

Subtract . . . . cc. Sn CI 2 solution, \ 

the equivalent I 

of.cc. \ 

Iodine solution i 
run back. / 


20 cc. Iron solution = net . . . cc. Sn CI 2 solution. 

SUMMARY 

but 20 cc. iron solution ^ = 100 milligrams of iron, 

hence 1 cc. Sn CI 2 solution = ... milligrams of iron. 

1 cc. iodine solution = ... milligrams of iron. 

The Process 

The Titration .—Place in a casserole 50. cubic centimetres of the stock 
solution; this represents .500 g. of ore. Add 20 cubic centimetres of pure 
concentrated hydrochloric acid, and boil the whole. While boiling, run in 
the tin solution from a burette until the iron solution becomes colorless. 
Now cool the colorless solution; when it is cold, estimate the excess of tin 
solution as follows: 

Add to the colorless solution 5 cubic centimetres of starch paste; then 
draw from a burette the iodine solution, drop by drop, until the blue color 
of iodine and starch appears. 

Computmg. —Find, as above directed, the average number of cubic 
centimetres of the tin solution actually needed for the iron. Knowing the 
value of one cubic centimetre of the tin solution in terms of metallic iron, the 
number of milligrams of metallic iron in 100 cubic centimetres of the iron 
solution tested may be obtained. 


Notes 

1. The strength of the standard iron alum solution should be such 
that one cubic centimetre contains 5 milligrams of metallic iron. The 
amount of the crystallized salt needed to furnish this amount of iron in a 
solution of 250 cubic centimetres is found by the following proportion: 
Molecular weight Molecular weight Grams Grams 

of iron : of iron alum :: of iron :: of iron alum 

111-8 964.57 1.250 10.775 







IRON 


43 


2. To ascertain that all the iron is oxidized to the ferric form, it is suf¬ 
ficient to show that ferrous iron is no longer present. For this purpose 
test the liquid by placing a drop of it in a drop of solution of potassium 
ferricyanide; no blue color should appear, for the reagent gives only a brown 
color with ferric salts, while it gives a deep blue precipitate (a variety of 
Prussian blue) with ferrous salts. But the solution of potassium ferricy¬ 
anide must be freshly prepared, as it decomposes upon keeping. 

3. Iodine is constantly evolving vapors which corrode the metal work 
of the balances. The weighing should therefore be quickly performed and 
the balance-case aired afterwards. 

Pure iodine is not absolutely necessary in this analysis. Iodine may 
be purified, when desired, as follows: 

Take about 3 grams of iodine, rub it in a mortar with a few crystals 
of potassium iodide, KI. Place the mixture between two watch-glasses, 
and gently heat so as to vaporize the iodine. Allow the glasses to cool, 
and when cold, scrape off the resublimed iodine from the upper glass. The 
potassium iodide purifies the iodine from chlorine and bromine. 

KI + Cl = K Cl -f- I. 

KI -h Br = K Br -f- I. 

4. Heat destroys the blue color of the iodide of starch, hence the so¬ 
lution must be cooled before adding the iodine. 



GOLD AND SILVER 

General Introduction 

In the majority of cases, a quantitative test for one of these metals 
involves a similar test for the other. 

The reason is to be found in the facts (first) that ores in which gold is 
the chief constituent usually contain also silver; (second) that ores in which 
silver is the chief constituent usually contain gold—sometimes in only min¬ 
ute amount; (third) that the various kinds of bullion—a term employed 
in a somewhat general way to designate almost any metal which contains 
larger or smaller quantities of gold or silver, or both at once, alloyed with 
if either produced directly or indirectly from ores manifesting the 

peculiarities mentioned, or else are so made up as to contain gold and silver 
at once. 

A gold coin, or a piece of gold jewelry, generally contains copper or 
silver, or both, in addition to the gold. 

A silver coin, or a piece of silver ware, usually contains copper alloyed 
with the silver. But in accordance with a preceding statement, it should 
be noted that there is also a minute quantity of gold present. 

In the statement of analyses following, in certain cases methods are 
given for-determining gold alone by an appropriate process, and in certain 
cases methods are given for determining silver alone by an appropriate pro¬ 
cess; but jn some cases reference is made to a process involving the deter¬ 
mination of both metals consecutivelv. 

The student ought to study carefully, at the outset, the Remarks which 
appear after the description of the dry assay of ores of gold and silver by the 
crucible process. 


44 


GOLD 


Principal Ores of Gold 

In the majority of rocks of the earth’s surface, especially silicioiis 
rock, gold exists—yet in most cases the amount present is so small that the 
metal cannot be extracted with profit. 

It is a general assumption that the gold generally exists in the metallic 
condition, called native gold (not the pure metal, but alloyed with metallic 
silver, and it may be other metals). 

But gold exists, sometimes in only minute quantity, in iron pyrites, 
and other so-called sulphurets—in such cases it is sometimes assumed that 
the gold itself is a sulphide. 

Gold—Synopsis of Methods 

Gravimetric Methods 

1. Metallic gold. Fresenius, 1:391. 

a. By ignition. 

b. By precipitation as metallic gold. 

Free the solution from nitric acid by evaporation. 

Precipitate with ferrous sulphate, oxalic acid, or zinc, cadmium, etc. 

c. By precipitation as auric sulphide. 

Precipitate by sulphuretted hydrogen in the presence of hydro¬ 
chloric acid. 

Wash, dry, and ignite. Weigh as metallic gold. 

Volumetric Methods 

1. Oxalic acid method. Sutton, 229. 

Add an excess of oxalic acid to precipitate the gold. 

Determine the excess by titration with standard permanganate 
solution. 

2. Iodine and thiosulphate method. Gooch and Morley’s method. Sut¬ 
ton, 229. 

Add a slight excess of potassium iodide to reduce the auric gold. 
Add starch paste. 

Titrate the solution with thiosulphate solution to the disappearance 
of the blue. Then add standard iodine until a faint rose color 
appears. * 


Dry Methods 

(These are applicable to bullion and to ores.) 

They are discussed more at length, further on, under the title Gold and 
Silver. 


45 


SILVER 


Classification of Ores of Silver 

a. Those containing the uncombined metal. 

Native silver. 

b. One practically free from sulphur. 

Keragyrite (horn silver), AgCl. 

c. Sulphuretted ores, some of them arsenical. 
Argentite (silver glance), AggS. 

Stromeyerite (silver copper glance), (AgCu) 2 S. 
Pyrargyrite, (ruby silver), Ag 2 SbS 3 . 

Stephanite, Ag 5 SbS 4 . 

Polybasite, (AgCu) 9 (SbAs)S 6 . 

d. Those containing large quantities of lead. 

Galena (lead Sulphide), PbS. 

Cerussite (lead carbonate). 


Principal Ores of Silver 


Keragyrite, horn silver, (Dana, 158) 
Silver chloride, 

AgCl. 

Cl 24.7 

Ag 75.3 


100 . 

Stromeyerite, (Dana, 50), 
Sulphide of silver and copper, 
Ag2S.Cu2S. 

S 15.8 

Ag 53.1 

Cu 31.1 


100 . 

Stephanite, (Dana, 143), 

A sulphide of silver and antimony, 

5Ag2S.Sb2S3. 

S 16.3 

Sb 15.2 

Ag 68.5 


100 . 


Argentite, silver glance, (Dana, 46) 
Silver sulphide, 

Ag2S. 

S 12.9 

Ag 87.1 

100 . 

Pyrargyrite, ruby silver, (Dana, 131) 
Sulphide of silver and antimony, 
3Ag2S.Sb2S3. 

S 17.8 

Sb 22.3 

Ag 59.9 

100 . 

Polybasite, (Dana, 146), 

A sulphide of silver and antimony 

9Ag2S.Sb2S3. 

S 15. 

Sb 9.4 

Ag 75.6 

100 . 


4G 








SI L ]' E R 


47 


Silver—Synopsis of Methods 

Gravimetric Methods 

1. Silver chloride. Fresenius, 1; 337. 

a. Wet method. 

Precipitate the silver as chloride with hydrochloric acid, in the 
presence of nitric acid. 

Weigh as AgCl. 

b. Dry method. Fresenius, 1: 339. 

Fuse the substance in a weighed glass tube. 

Convert it into silver chloride by passing a current of chlorine gas 
over the fused mass. 

Repeat this process until the weight of the tube, plus that of the 
chloride, is constant. 

2. Silver sulphide. 

Precipitate as sulphide with hydrogen sulphide gas in the pres¬ 
ence of an alkaline nitrate. 

Collect on a weighed filter, wash, and dry at 100°. 

Weigh as silver sulphide, AggS. 

Dissolve the sulphur (contained in the precipitate) with carbon di¬ 
sulphide. 

Wash, dry, and weigh again. 

Subtract the second weight from the first and compute. 

3. Silver cyanide. Fresenius, 1: 341. 

Neutralize any free acid with potassium carbonate. 

Precipitate as cyanide with potassium cyanide in the presence of 
nitric acid. 

Collect the precipitate on a weighed filter, wash, and dry at 100°. 
Weigh as silver cyanide, AgCN. 

4. Metallic silver. 

a. Dry method. 

(1) Reduce the salt, by direct ignition in a weighed porcelain cru¬ 
cible. 

(2) Reduce the salt, by heating in a current of pure hydrogen. 

b. Wet method. 

Convert the silver to silver sulphate by treatment with sulphuric 
acid. 

Reduce to metallic silver by metallic cadmium. 

Wash the precipitated silver, dry, and ignite. Weigh as Ag. 

Volumetric Methods 

1. Sodium chloride methods. Sutton, 325. 

a. With chromate indicator. Mohr’s method. Sutton, 152. 

Precipitate the silver as chloride until red silver chromate appears. 

b. Gay-Lussac’s method. Sutton, 327. 

Precipitate the silver as chloride with a strong standard solution 
of sodium chloride (called ‘hhe unit solution”) until nearly all 
the silver is precipitated. 




(8 


THE ASSAY-BOOK 


Finish the titration with a weak standard solution of salt (called 
‘‘the decimal solution”—it is of one-tenth of the strength of the 
unit solution). 

Make sure of the end-point by use of a decimal solution of silver. 

2. Starch-iodide method. Pisani’s method. Si^tton, 326. 

Add a solution of blue starch-iodide to a neutral silver solution 
until the blue color is just permanent. 

3. Ammonium sulphocyanate method. Volhard’s method. Sutton, 155. 

Add a standard ammonium sulphocyanate solution, using a ferric 
indicator, until the appearance of red ferric sulphocyanate. 

Dry Methods 

1. For solutions. 

Evaporate the solution to dryness. Treat the residue as described 
further on for ores. 

2. For alloys. 

Wrap a portion of the alloy in pure lead,(it may be with addition of 
pure silver), and then separate the silver by cupellation, as described 
later under the title Gold and Silver. 

3. For ores. 

Subject the ore to fusion, cupellation and quartation, as described later 
under the title Gold and Silver. 

Silver—Gravimetric Test—Chloride Method 

Outline of the Process 

(a) Roll the coin into a thin ribbon. 

(b) Dissolve the bullion in nitric acid. 

(c) Precipitate the silver with hydrochloric acid 

(d) Weigh the metal as chloride. 

this is the ordinary, and well-known, .process of gravimetric determina¬ 
tion of silver: therefore its detailed description need not be repeated at 
length here. 

Sliver—Volumetric Process—Gay-Lussac^s Method 

This is the approved process employed in the government assay offices 
of most of the great nations of the world. From time to time it has been 
carefully studied and subjected to slight modifications to promote its speed 
and accuracy. 

In government offices, the analyst has usually the great advantage of 
knowing in advance very nearly the exact amount of silver in the bullion 
tested. 

If the approximate amount of silver in a given specimen of bullion is 
not known by the investigator, it may be learned by a preliminarv test. 

Outline of the Process 

(a) I repare a standard solution of common salt (1 cc. corresponds to 
10 mg. metallic silver). This may be called “the unit solution.” 



S I L V E R 


49 


(b) Prepare a “ decimal ” solution ot‘ common salt (1 cc. represents 1 
mjT. of metallic silver). 

(c) Prepare a “ decimal ” solution of silver nitrate (1 cc, corresponds 
to 1 mg. of metallic silver). 

(d) Dissolve the bullion in a test bottle, 

(e) Titrate the bidlion solution, using as described the two salt so¬ 
lutions and the ‘'decimal” silver solution. 

The Standard Solutions 

(a) The unit salt solution. —Weigh about 6 g. of pure chloride of so¬ 
dium. Heat the salt carefully to expel moisture; but do not carry the heat¬ 
ing to the point of fusion of salt. 

Weigh of this dried salt 5.4207 g. Dissolve this quantity in distilled 
water and make the solution to the volume of 1 liter. 

This solution is intended to be of such a strength that 1 cc. corresponds 
to 10 mg. of metallic silver. 

(b) The decimal salt solution. —From the unit salt solution draw 50 cc.; 
then dilute this quantity to the volume of 500 cc. 

This solution is intended to be of such a strength that Icc. corresponds 
to 1 mg, of metallic silv^er. 

(c) The decimal silver solution. —Weigh as exactly as possible .500 g. 
of fine silver. Dissolve this quantity in from 2 to 3 cc. of concentrated nitric 
acid, 1.20 sp. g. Dilute this solution to the volume of 500 cc. 

If this solution is correctly made, 1 cc, of it contains exactly 1 mg. of 
metallic silver. 

The Standardizing 

Note. —Although the unit salt solution is intended to have the exact 
relation to pure silver stated above, its real working power must be found 
by experiment. 

Testing the unit salt solution. —W'eigh as exactly as possible 1.003 g. 
of fine silver; place the metal in a test bottle with 5 cc. of pure nitric acid, 
1.20 sp, g.; heat the mixture gently on a hot plate until the metal is com¬ 
pletely dissolved; by means of a glass tube, blow nitrous vapors from the 
bottle; allow the bottle to cool to the temperature of the air. 

To the silver solution add exactly, from a pipette, 100 cc. of unit salt 
solution. 

Place the glass stopper in the bottle; place the whole in the black bag; 
shake the whole thoroughly, to coagulate the silver chloride; remo^'e the 
bottle from the bag ; carefully lift the stopper, washing down the solution 
and precipitate, with distilled water. The liquid above the precipitate 
should be as clear as water. 

From the proper burette draw decimal salt solution to the extent of 
.5 cc.; it should produce a slight cloud of silver chloride. Shake the bot¬ 
tle in its l)ag. llepeat additions of decimal salt solution, .5 cc. at first, 
and later one or two drops at a time until, after the various shakings, the 
point is reached at which addition of one or two drops has given no milki¬ 
ness. 



50 


THE .1 SSAY -BOO K 


Consider these final two drops as an excess of decimal salt solution, 
and take as the reading the number of cc. used, the two drops being sub¬ 
tracted. 

If for any reason the proper point has been passed, draw in exactly 
2 or 3 cc. of the decimal silver solution—at all events, a slight, but known, 
excess. Now go back and titrate again with decimal salt solution until the 
true point is reached. 

Compute the value of unit salt solution in terms of metallic silver as 
follows: 

First—to the amount of silver originally weighed, add an amount rep¬ 
resented by the quantity of decimal silver solution run back. 

Second—divide by 10 the number of cc. of decimal salt solution used. 
Add the result to the number of cc. of unit salt solution employed, that is 
100 . 

Third—divide the total weight of silver involved, by the number of cc. 
of unit salt solution employed; the quotient represents the real working 
value of the unit salt solution per cc. expressed in terms of metallic silver. 

The Process 

1. — Weighing the bullion (previously rolled).—C'ompute as nearly as 
possible on the basis of information what weight of bullion represents 1 g. 
of pure silver; then weigh exactly this amount. 

If the analyst has not satisfactory information as to the amount of 
silver present in the bullion tested, he may make a quick direct test with 
unit salt solution; upon the results of this test he may compute how much 
bullion to employ to yield very nearly 1 g. of pure silver. 

2. Dissolving. —Place the weighed bullion in a test bottle with 5 cc. 
of pure nitric acid 1.20 sp. g.; heat the mixture gently^ on a hot plate until 
the metal is completely^ dissolved; by means of a glass tube, blow nitrous 
vapors from the bottle; allow the bottle to cool to the temperature of the 
air. 

Continue the operation exactly as described above under ‘‘Standard¬ 
izing,” that is, add 100 cc. of unit salt solution; shake; add decimal salt 
solution; if necessary, add decimal silver solution; again add decimal salt 
solution until the end-point is determined. 

3. Computing. —Compute the amount of silver in the bullion, not in 
terms of per cent, but in terms of per thousand. (For example, if a portion 
of bullion shows 92.5 per cent of silver, report it as 925. thousandths). 

Notes 

In titrations such as those described, there may be reached a stage 
called by Mulder “the neutral point”; at this point, if the clear solution is 
divided into two halves, the one half will give milkiness upon addition of 
decimal salt, and the other half will give milkiness upon addition of decimal 
silver solution (at this stage the solution contains silver chloride dissolved 
in sodium nitrate—or at all events there exists some kind of equilibrium 
of the several salts present). 








SI LV E It 


51 


But the ordinary analyst rarely hits this condition—practically, he in¬ 
tends to introduce a slight excess of salt and make allowance for it. Fre- 
senius, 1:343. 

Silver—Volumetric Process—Potassium Sulphocyanate 

Method 

Outline of the Process 

(a) Prepare a standard solution of potassium sulphocyanate. 

(b) Prepare a ferric solution (for use as an indicator). 

(c) Prepare a standard solution of silver nitrate from fine silver. 

(d) Dissoh'C the bullion to be tested and then titrate it by the stan¬ 
dard potassium sulphocyanate solution, using the ferric solution as an 
indicator. 

Supplies 

Silver bullion. Fine silver. Potassium sulphocyanate. Nitric acid. 
Ferric alum. Burettes. 

The Standard Solutions 

1. The sulphocyanate solution. —Weigh 5 g. of the potassium sulpho¬ 
cyanate; dissolve in water; then dilute to 1000 cc. 

2. The ferric solution. —Dissolve 10 g. ferric alum in 30 cc. water. Add 
to the solution 5 cc. nitric acid. 

3. The silver nitrate solution. —Weigh two portions of “fine” silver, 
each about .250 g. Treat each as follows: dissolve the metal in about 2 cc. 
concentrated nitric acid, diluted with about 5 cc. of water. After solution, 
add 10 cc. concentrated nitric acid, sp. g. 1.20. Boil the solution until 
fumes of nitrous compoi:\nds are expelled. 

Add 5 cc. of the ferric solution (as the indicator). 

4. Standardizing the sulphocyanate solution. —Treat each silver so¬ 
lution further as follows: Draw from a burette into it sulphocyanate solution 
until a faint red tinge can be detected. 

5. Computing. —Compute the value of the solution in terms of me¬ 
tallic silver per cc. 

• The Process 

1. ]Yeighing the bullion, (previously rolled).—Weigh two portions, 
each .250 g. of the bullion to be tested. Treat each as follows* 

2. Dissolving. —Dissolve the weighed portion in 5 cc. nitric acid; l)oil 
the solution until all the nitrous fumes are expelled; cool the liquid; then 
dilute to 50 cc. 

3. Titrating. —Add 5 cc. of the ferric solution, as an indicator, and 
then draw in sulphocyanate solution to the appearance of the red tinge. 

4. Computing. —Make the proper computations expressing the amount 
of silver as thousandths of the original bullion 

Note 

This process is practicable in presence of copper in quantity below 
70 per cent; if the amount of copper is greater, its relative quantity may 
be reduced by addition of fine silver in known amount. 




52 


THE ASSAY-B00K 


Silver—Dry Process—Cupellation Process 

This process is applicable to known alloys containing silver aiul it may 
be other base metals, but containing very little gold. If considerable gold 
is present (33 per cent and upwards), silver must be added and a parting 
process must be employed as described. 

The student should carefully examine the Remarks which appear fur¬ 
ther on under the title Gold and Silver together—Crucible Process, page GO. 
In these remarks the details of the chemical changes associated with cupel- 
lation and parting are discussed at some length. 

Outline of the Process 

Wrap a weighed portion of the ribbon in pure metallic lead and cupel 
the mixture. The cupellation removes the lead and leaves the silver as a 
“prill”—a small button. (Rut there may be a minute amount of gold pres¬ 
ent.) 

Part the prill with nitric acid. Weigh the gold, if any. (Prove that 
the residual material is gold—first, by inspecting it under the microscope; 
second, by the purple of Cassius test. 

The Process 

1. Weighing, etc .—Weigh about .200 g. of the rolled bullion. Wrap 
this in about 5 g. of assay-lead. (But the amount of lead ought to be about 
20 times the amount of copper believed to be present.) 

2. Cupelling .—Select a cupel whose weight is approximately that of 
the lead button. 

Heat the cupel thoroughly in a muffle. 

Wrap the lead button in a piece of filter paper; then place it in the 
heated cupel, and place over it and near it a somewhat large piece of charcoal. 
The lead should soon fuse; and the charcoal and the carbon of the filter paper 
should not only help to heat the button, but should reduce the oxide on 
its surface. 

When the button appears to be red hot and to display a clean metallic 
surface, remove the charcoal. • 

Allow the operation to continue, regulating the fire from time to time, 
so that the button may neither become too hot nor too cold, and so that the 
current of air may be sufficient to oxidize the lead (which is, in a proper 
sense, burning), but not sufficient to chill the surface of the button and so 
stop oxidizing. 

W'atch the gradual diminution in size of the button. 

W hen the button has so far reduced that only a prill is left, watch for 
the brightening of the prill (this sudden brightening is followed by a duller 
appearance and sometimes a display of prismatic colors on the surface). 

Carefully remove the cupel from the furnace. Allow the whole to cool. 

When the cupel is cool, remove the prill with a pair of tweezers or with 
the point of a knife blade. Remove any adhering bone-ash from the prill 
(sometimes this is best done by fusing the prill on a piece of charcoal before 
the blow-pipe. 



S I L T' E R 


53 


Weigh the prill. 

• 3. Parting the prill .—Place the prill in a 6-inch test-tube. Add 2 cc. 
of dilute nitric acid. Warm the mixture gently, so as to dissolve silvei’. 
But the solution must not proceed too rapidly. 

Carry the process of dissolving silver to completion, using repeated 
portions of dilute nitric acid; decanting the silver nitrate formed; toward 
the end using concentrated nitric acid, to remove the last portions of silver; 
finally washing the residual gold, even though its amount be very small, 
with water. 

4. Weighing the gold .—(The operation next described has for its pur¬ 
pose the transfer of even minute quantities of gold without loss to a porce¬ 
lain crucible in which the gold is to be heated.) 

Provide a casserole full of water. Provide a small porcelain crucible. 

Fill wdth water to the very top the test-tube containing the gold; cover 
the test-tube with the little crucible as with a cap; quickly invert this system 
so that the crucible is in the downward position and the test-tube is in the 
upward position, as of a bell-glass nearly full of water; now the gold should 
fall through the water into the crucible. Transfer the system to the casserole 
so that the little crucible is at the bottom of the casserole and the junction of 
crucible and test-tube is under water; raise the test-tube slightly; pass it—its 
mouth still under water—to one side; then lift the porcelain crucible—still full 
of water and with the gold at the bottom—out of the casserole. Carefully 
pour away the water in the crucible so as not to disturb the gold. Care¬ 
fully dry the gold in the crucible and finally heat it to a red heat. Cool 
the crucible and contents. 

Weigh the gold. 

5. Testing the gold .—After weighing the gold, examine it under a 
microscope. 

Transfer the gold to a small porcelain crucible; to it add 1 or two drops 
of aqua regia; add the drops of solution carefully and without agitation 
to a test-solution containing the proper iron and tin salts for this purpose; 
the gold should give a coloration or precipitate of purple of Cassius. 




GOLD AND SILVER TOGETHER 

Gold and Silver in Bullion—Synopsis of Methods 


Gravimetric Method 


1 . Nitric acid method. 

Treat an alloy with nitric acid to dissolve all metals (in certain cases 
including silver), leaving gold imattacked. 

Wash the residue, ignite it, and weight it as metallic gold (if necessary, 
test the weighed residue for silver). 

Dry Methods ' 

1. Cupellation method. 

Alloy the bullion with a computed amount of silver. 

Wrap the alloy wdth assay-lead and cupel. 

Part the prill. 

Alloys containing gold may contain beside the gold: 

1 st. Base metals which are easily oxidizable, as lead, copper, etc. 

2nd. Noble metals which are not easily oxidizable, as silver, and also 
platinum and the platinum metals, so-called. 

Under certain conditions, treatment with nitric acid may remove the 
base metals (except, perhaps, antimony and tin), and even silver, but will 
leave gold, platinum, and some of the platinum metals undissolved. 

Tlie cupellation process, if properly conducted, removes the base 
metals, leaving an alloy of gold, silver and platinum and, it may be, the 
platinum metals. 

Gold and Silvcf m an Ore—Synopsis of Methods 

1 . Crucible method. Brown, 170. 

Fuse the ore in a crucible with the proper fluxes, including litharge 
(lead oxide), intended to furnish metallic lead, which shall, in the 
fusion, collect the gold and silver in the ore, and then form a button. 

Cupel the lead button, thus removing the lead and leaving a smaller 
button or ''prill” containing gold and silver. 

Alloy the prill with a computed amount of pure silver. (Alloys con¬ 
taining 33 per cent of gold and upwards do not permit of complete 
removal of silver by nitric acid.) 

Part the new' prill with nitric acid. 

Weigh the gold left. 

(The weight of the first prill has been learned; the weight of the gold 
has been learned; the difference between these weights is the weight 
of the silver.) 


54 


GOLD AND SILV ED TOGETHER 


,55 


I 

2. Scorification method. Brown, 139. 

Scorify the ore with metallic lead, borax-glass, or other fluxes. 

Cupel the lead button. 

Part the prill. 

3. Chlorination method. Brown, 234. Ricketts, 194. Kustel, Extrac¬ 
tion of gold by chlorination, 136. 

Pass chlorine gas through the moistened ore (soluble chloride of gold 
is formed.) 

With water, wash the chloride of gold from the gangiie. 

By means of ferrous sulphate, precipitate metallic gold from its solution. 
Wrap the gold in assay-lead and cupel the product. 

4. Amalgamation method. Brown, 221. 

Grind the ore with water, potassium cyanide and mercury. Strain out 
the mercury, now containing gold. Scorify and cupel the gold amal¬ 
gam, 

5. Pan test. Brown, 225. 

Weigh from 3. k, to 15. k. of ore; pulverize it. 

Wash it in a current of water, using a large pan (the purpose is to wash 
away lighter portions of ore, but to retain the gold). 

Scorify the residual gold with assay-lead. 

Cupel. 

NOTE.—Certain gold ores may contain gold in the form of scales, 
which can be removed from the powdered ore by sifting. 

In such cases, the scales are tested as Inillion; the powdered ore is 
tested as an ore. 


Gold and Silver—Test of Gold Bullion Wet Process 

Outline of the Process 

(a) Make a preliminary test for silver. 

(b) Alloy the bullion with the silver if necessary. 

(c) Treat a portion of bullion with nitric acid, 

(d) Weigh the metallic gold. 

The Process 

1. PTelimiTiciT]/ test .—Weigh about .100 g. of thin ljullion; to it add 
3 cc. of aqua regia (about 1 cc. of nitric acid, and 2 cc. hydrochloric acid). 
Gently warm the mixture. 

When the bullion is completely dissolved or completely disintegrated, 
dilute the solution with water. If silver is present, in considerable quan¬ 
tity, it will appear as silver chloride. 

Filter the mixture and learn the weight of silver chloride as usual; from 
this weight, compute the per cent of silver in the bullion. 

Compute how much silver, if any, must l)e added in order to make the 
amount of silver and base metal together equal to twice the weight of gold. 





50 


THE ASS .1 Y -BO 0 K 


2. Adding silver. —Weigh about .200 g. of thin bullion; place it on 
charcoal together with the required quantity of silver; fuse the two; roll 
the alloy to a thin ribbon. 

3. Parting. —To the ribbon, in a test-tube, add 1 cc. of water and 1 cc. 
of concentrated nitric acid; warm the mixture. When action has practi¬ 
cally ceased, decant the solution. Then, to the gold, add 2 cc. of concen¬ 
trated nitric acid, and warm again—hoping thus to remove the last traces 
of silver. 

Wash the gold several times wdth water. 

4. Drying and weighing the gold. —Transfer the gold to a porcelain 
crucible; dry the whole and then ignite it gently; cool the crucible and con¬ 
tents. 

Weigh the gold as accurately as possible. 

5. Computing. —Compute the gold as thousandths of the bullion. 

Gold and Silver—Test of Gold Bullion—Cupellation Process 

Outline of the Process 

(a) Alloy the bullion with a computed amount of silver. 

(b) Wrap the alloy with assay-lead and cupel. 

(c) ‘‘Part” the “prill.” 

The Process 

1. Adding silver. —Weigh about .200 g. of the bullion. Compute the 
probable amount of silver to be added (so as to give an alloy containing 
two parts silver to one part gold); alloy the bullion on charcoal, before the 
blow'pipe, with the exactly weighed amount of fine silver. 

2. Cupelling. —Wrap the alloy in a portion of assay-lead judged to 
be about 20 times the weight of copper believed to be present. 

Cupel the mass; w^eigh the prill; flatten it on a clean anvil. 

3. Parting. —To the prill in a test-tube add a few cc. of dilute nitric 
acid; w'arm the mixture. When action has practically ceased, decant the 
solution. Then, to the gold, add 2 cc. of concentrated nitric acid, and w'arm 
again^—hoping thus to remove the last traces of silver. 

Wash the gold several times with w'ater. 

4. Drying and weighing the f/oW.—Transfer the gold to a porcelain 
crucible; dry the whole and then ignite it gently; cool the crucible and 
contents. 

Weigh the gold as accurately as possible. 

5. Computing. —Compute the gold as thousandths of the bullion. 

From the weight of the prill, subtract the weight of added silver; the 

difference is the weight of gold and silver in the bullion. 

From the total weight of bullion, subtract the weight of gold and silver; 
the difference is the weight of base metal in the bullion. 

From the weight of gold and silver in the bullion, subtract the weight 
of gold; the difference is the weight of silver in the bullion ; compute it 
in thousandths. 







GOLD AND SI LV EU T 0 0 E T II E E 


57 


fn 


Gold and Silver—Test of an Ore—Crucible Process 


Brown, 170 
Outline of the Process 

(a) Make the necessary preliminary tests. These are: 

—Test the reducing power of argol or other reducing agent, 

—Test the litharge for silver and gold. 

—Test the assay-lead for silver and gold. 

(b) Pulverize the ore. 

(c) Fuse the ore in a crucible with the proper fluxes. 

Cool the crucible; then break it, and secure the lead button. 

(d) Cupel the lead button. 

(e) Part the prill. 

Supplies 

Hessian or Battersea sand crucibles. Cupels. 

Litharge. Sodium carbonate or hydro-sodium carbonate. Borax. Ar¬ 
gol. Common salt. Pure assay-lead. Fine silver. 


The Preliminary Tests—the Blank Tests 

1. The reducing 'power of argol. —Brown, 98, 121. Weigh the follow’- 
ing substances: 


Sodium bicarbonate. 

30 g. 

Litharge, 

30 g. 

Argol, 

2 g. 

Salt, 

10 g. 


Mix all the substances, except the salt, in a wedgwood mortar or on 
a piece of paper. Place the mixture in a sand crucible of proper size. Place 
the salt on top of the mixture, and pack the whole down with a wedgwood 
pestle. 

Place the crucible and contents in a hot fire. Cover the crucible with 
an iron cover. When the mixture is thoroughly fused (requiring 15 or 20 
minutes) place the hot crucible in a larger crucible to cool. 

When the smaller crucible is thoroughly cooled, break it with a hammer, 
detach the lead button from the slag. Weigh the lead button. Since 
2 g. of argol were used, and since an excess of litharge was present, the re¬ 
ducing power of 1 g. of argol is apparent. 

The reducing power of other agents ma}'^ be determined in similar 
manner. 

2 . Testing litharge for silver and gold. —Browm, 116. Weigh the fol- 


substances: 


Sodium bicarbonate. 

120 g. 

Potassium carl)onate. 

5 g. 

Litharge, 

120 g. 

Argol, 

4g. 

Salt, 

20 g. 





.58 


THE ASS A Y -BOO K 


Place the charge in the crucible, and conduct the fusion; then remove 
the lead button, exactly as described in the process of testing the reducing 
power of argol, above. 

Cupel the lead button as described below in the process for testing an 
ore. Later, weigh the prill and part it, exactly as described below in the 
process for testing an ore. 

3. Testing assay-lead for silver and gold. —Brown, 118. Weigh 60 g. 
of the assay-lead. Place the metal in a scorifier, and cover it with a few 
fragments of borax glass. Scorify the whole by a process described later, 
the purpose being to reduce the weight of the lead to a quantity suitable for 
cupellation. 

Remove the lead from the scorifier; cupel it; part the prill. 

The Process 

1. Pidverizing the ore .—Weigh approximately 200 g. of ore. If the 
latter is in lumps, carefully select such as are fairly representative of the 
whole lot supplied. It is of the utmost importance that the portion tested 
shall be a just sample of the material furnished. 

Break the lumps down to as fine a powder as practicable, in an iron 
mortar; then reduce the whole to a still finer powder in a wedgwood mortar. 

2. Preparing the charge .—Weigh the following substances: 


Powdered ore 

30 g. (or 29.166 g.) 

Sodium carbonate 

30 g. 

Litharge 

60 g. 

Borax 

30 g. 

Argol 

4 g. 

Common salt 

20 g. 


Place all the materials except the salt in a wedgwood mortar and mix 
them carefully. Transfer the mixture to a suitable sand crucible. Grind 
the salt in the same wedgwood mortar (so as to rinse the mortar); then put 
the salt in the crucible on top of the other materials. By means of a 
wedgwood pestle, pack the whole charge in a somewhat compact form 
in the crucible. 

3. Fusing .—Place the crucible in the fire; cover it with an iron cover; 
continue the fusion for 15 or 20 minutes, or until the whole mass is red-hot, 
thoroughly liquid, and has ceased to give off any considerable quantity of 
gas or vapor in the form of bubbles. 

When the fusion is judged to be completed, remove the crucible and 
contents, placing the whole on an iron plate to cool. 

When the crucible and its contents have thoroughly cooled so that the 
whole may be handled, break the crucible with a hammer and examine the 
contents. These should be practically three; (First) on top, a layer of salt; 
generally distinct from the other materials. (Second) the slag; this is 

generally dark in color but it should be uniformly glassy in appearance_ 

should be free from lumps of unassimilated material. (Third) the lead 
button; this should be a well rounded disc-like piece, regular in form. 

There should be no small globules of lead distributed through the slag. 




GOLD AND SILVER TOGETHER 


59 


The button should not have lying above it either a brilliant layer (look¬ 
ing like lead but separating from the button), nor yet a dark, brittle, gran¬ 
ular layer of so-called “regulus.” 

Hammer the button, on a clean anvil, into a more or less rectangular 
form. (The principal purpose of the hammering is to detach fragments 
of slag, which latter are brittle.) 

The button should be soft and malleable. Brittleness in the button 
may indicate presence of sulphur or of some other substance, as arsenic, 
antimony, or something else. 

In any event, weigh the button. (It may advantageously weigh from 
15 to 30 g.) 

4. —Cupelling the lead button .—Select a cupel whose weight is approxi¬ 
mately that of the lead button. 

Cupel the lead button as already described, page 52. 

Weigh the prill. 

5 . —Parting the prill .—If the prill is believed to have less than 33 per 
cent of gold, part it immediately with nitric acid. 

If the prill has 33 per cent and upwards of gold, alloy it with a com¬ 
puted amount of fine silver, so as to reduce the relative amount of gold to 
about 30 per cent. Then part this prill with nitric acid, as usual. 



REMARKS 


1. Alloys of gold and silver .—Native gold of Calilornia is said to average 
from 875 to 885 thousandths fine; that of Australia is said to average from 
960 to 966 thousandths fine. 

Fine gold (that is, gold which is perfectly pure) is nearly as soft as lead; 
hence, it is rarely, if ever, used for coin or jewelry—it is usually alloved with 
copper or silver, or both. 

An alloy containing gold in considerable amount and copper in small 
amount, is at once harder than pure gold, and more easily fusible. 

An alloy containing gold in large amount and antimonv or lead in small 
amount, is brittle. 

The proportion of gold in an alloy is usually expressed in thousandths; 
it IS also often expressed in twenty-fourths called carats (but the carat sys¬ 
tem may express so small a fraction as l-768th, for each carat is divided into 
four assay grains and each assay grain into 8 eighths; thus pure gold may 
be called 24 carats fine, or 96 grains fine, or 768 eighths fine.) 

The gold coin of Great Britain contains II parts of gold to 1 part of copper; 
hence it is 22 carats fine or 916.66 thousandths fine. 

1 he gold com of the United States and of France contains 9 parts of gold 

and 1 part of copper; hence it is 21 carats 2f grains fine or 900 thousandths 
fine. 


Pure silver is too soft to resist much wear; hence it is rarely, if ever, 
used for coin or jewelry—it is usually alloyed with copper. 

An alloy containing silver in considerable amount and copper in small 

amount, is at once harder than pure silver and it retains practically the 
whiteness of the latter. 

The silver coins of Great Britain contain 92 5-10 per cent of silver and 
7 5-10 per cent of copper; hence the alloy is 925 thousandths fine. (It is 
called sterling silver.) 


The silver coins of the United States and of France contain 90 per cent 
of silver and 10 per cent of copper; hence they are 900 thousandths fine. 

The term 'coin” with respect to silver is applied only to pieees of the 
higher denominations; pieces of lower denominations, as well as those con¬ 
sisting largely of copper, or of bronze, or of nickel, are called "tokens.” 

•The proportion of silver in an alloy is usually expressed in thousandths; 
the term carat and its congeners are not applied to alloys of silver. 

^on.—Ores are often weighed by the avoirdupois ton of 

2000 pounds, while gold and silver are weighed by the troy ounce. Now 

2000 pounds of 7000 grains each equal 14,000,000. grains. But 1 ounce 

troy equals 480 grains. 14,000,000.480 = 29,166. Hence 1 part of 

noble metal in 29,166. parts of ore, represents 1 troy ounce of noble metal 
in 1 avoirdupois ton of ore; therefore 1 milligram of gold or silver in 29 166 
pams of ore, represents 1 ounce of noble metal to the ton of ore Thus 
It has risen, that a weight representing 29.166 g. is called 1 assay ton- and 

of course from 1 assay ton a weight of 1 milligram mav be called 1 assav 
ounce. ‘ ‘ j 


00 


R E 71/ A R K S 


()1 


3. Substances used in the fusion assay .—The coiiniionost substances 
employed are litharge (lead oxide, PbO,), sodium carbonate, argol (an im- 
])ure mixture containing potassium tartrate), borax, common salt. 

Litharge is used for three reasons: 

First, it is a valuable flux by itself, because it fuses readily; second, 
it forms an easily fusible silicate with silicon dioxide, an important constit¬ 
uent of most ores; third, in the process of fusion, with reducing agents like 
argol, it liberates metallic lead in very fine particles. These particles of 
lead, in a molten condition, fall through the fused mass in a kind of shower; 
they very readily alloy themselves with gold and silver in the charge; to¬ 
gether they fall to the bottom of the crucible, forming there a button of lead. 
(This button is subsequently cupelled.) 

Litharge should be pure to the extent of being free from everything 
but gold and silver; it is difficult to obtain litharge entirely free from these 
metals; their amounts in the sample of litharge used must be determined 
by a blank test and subsequently allowed for. 

Sodium carbonate acts in a manner somewhat similar to litharge; that 
is, it forms a fusible silicate with silicious matter of the ore. 

Of course, in forming the silicate, carbon dioxide is liberated. It mani¬ 
fests itself in bubbles of gas which escape from the ‘‘melt.” While escaping, 
these bubbles help to agitate the melt—and in so doing they advantageously 
mix the various materials together. But they ought to be completely ex¬ 
pelled before the crucible is removed from the furnace. 

Borax is used sometimes in the form of powder derived from the crys¬ 
tallized substance, Na 2 B 407 . IOH 2 O; sometimes in the form of borax 
glass (the latter is borax from which the water of crystallization has been 
expelled). 

Borax, when in a state of fusion, • is able to dissolve metallic oxides; 
the so-called bases. When the crucible charge is melting, the borax 
dissolves those oxides which are in the ore, forming with them double borates 
of varying constitution, but fusible. Among the most important of these 
oxides are those of iron, aluminium and calcium. The following may be 
accepted, in a general way, as a typical equation: 

Na 2 B ^67 -h FeO = Na 2 FeB 408 

If borax is used, the heat drives off its water of crystallization; in this 
process, the melt sometimes froths; but after complete expulsion of water, 
the melt subsides into a thin liquid mass. 

Litharge and sodium carbonate may be looked upon as basic fluxes: 
they combine with acid material, principally silicic oxide, of the ore. Borax 
may be looked upon as an acid flux; it combines wth basic materials of the 
ore, as already described. If a given ore is largely silicious, it may be nec¬ 
essary to increase the relative amount of basic fluxes; if an ore is largely 
basic it may be necessary to increase the relative amount of acid flux. 

Argol or argots is an impure potassium tartrate. It may be a mixture 
of tartrates, or it may be a calcium-potassium tartrate. It is obtained from 
^vine—settling as a deposit at the bottom of the casks. Its precipitation 
from solution is incidental to the formation of alcohol by fermentation. 





62 


T II E .1 ,S S .1 Y -BOOK 


Argol is used as a reducing agent; when heated in the melt it liberates 
carbon in a finely divided condition. (It also forms potassium carbonate.) 
The carbon reduces the litharge, setting free metallic lead, as already stated. 

Salt is placed upon the top of the charge before melting. During the 
melting, the salt fuses; but it usually retains its place above the other ma¬ 
terials and without considerably diffusing among them. Although some of 
the salt is vaporized at the high temperature of the process, the most of it 
is found as a layer above the slag after the crucible has cooled. The salt 
appears to act during fusion as a kind of cover, which keeps the melt at a 
higher temperature than it would otherwise attain. Further, it is believed 
to prevent loss of silver by volatilization. 

4. Scorijication is an oxidizing process—in the main. It is usually per¬ 
formed in a shallow earthen dish; l)ut the clay of which the scorifier is made 
must be carefully selected as one which powerfully resists fluxes. 

bcorification may be applied to a lead button for the purpose of diminish¬ 
ing the quantity of lead preliminary to cupellation. Of course the process 
must not be carried on too long—there is danger of diminishing the lead un¬ 
duly, and there is danger of perforating the scorifier by reason of the fusible 
lead silicate formed. 

bcorification may be applied to an ore mixed with metallic lead. In 
this case volatile materials of the ore, like sulphur, arsenic, antimony, etc., 
may be oxidized and vaporized; the oxide of lead formed coml^ines with 
silicious substances to form a fusible slag; unoxidized lead combines with gold 
and silver to form a button suitable for cupellation. 

5. Cupellation is a very ancient process, whose purpose is to remove 
the base metals of a specimen of bullion, and to leave a prill of gold and silver 
in a form such that it may be removed and;weighed. 

Cupellation is performed in a cupel made of bone-ash. The bone-ash 
does not melt at high temperature nor does it combine with oxide of lead to 
form a slag; on the contrary it allows the melted oxide of lead to sink into its 
minute pores. At the same time the oxide of lead may 'carry into the cupel 
with it a moderate amount of oxide of copper (about 1-20 of its weight) 
but it does not usually carry all the copper in—a small portion called the 
“surcharge” is left in the prill. 

In cupellation, about 2 or 3 per cent of the metallic lead vaporizes as 
such; the lead vapor is usually oxidized in the air above the cupel. 

Cupellation is usually conducted in an arched clay chamber, called a 
“muffle,” surrounded by burning fuel, except at the front. The heat re¬ 
flected or radiated from the roof of the muffle, is an important factor in keep- 
ii^S fhe upper portions of the button at a high temperature, and so favoring 
cupellation. 

If the temperature of the muffle, and thence of the button, falls to a 
certain point, the button may become coated with oxide of lead which thus 
no longer fuses; now the operation of cupellation stops. But it may be re¬ 
newed, by increasing the heat of the fire and at the same time by placing a 
small piece of charcoal directly upon the button in the <?upel. Now the 
charcoal burns; its combustion raises the temperature of the button; the 




R E M .4 R K S 


03 


carbon itself reduces the oxide of lead which has formed a film on the button. 
When these results have been secured, the charcoal may be removed, and the 
operation of cupellation may proceed. 

The end of the process is reached when the lead and other base metals 
have been completely removed—being volatilized into the air or else oxi¬ 
dized and absorbed as oxides into the cupel. 

The end-point is determined by the so-called “brightening” of the silver. 

Pure silver, when melted in air, dissolves about 22 times its volume of 
oxygen gas. When such silver begins to cool in the air, an exterior crust is 
first formed. Soon the general lowering of temperature leads to the ex¬ 
pulsion of the oxygen gas; the latter escapes through the outer crust with 
violence, carrying with it some molten silver from the interior; this silver 
cools in filaments. The button is said to have “sprouted” or “vegetated.” 

6 . Parting, that is, separating gold and silver by acid, is usually accom¬ 
plished by nitric acid. The acid acts best on bullion containing 2 1-2 to 3 
parts of silver to 1 part of gold (30 per cent to 25 per cent of gold); if less sil¬ 
ver is present, it is not completely dissolved by the acid; if more silver is 
present, the gold is liable to be left in so spongy a condition that it easily 
breaks into fragments, of which some are liable to be lost (it is always pref¬ 
erable, in parting, to have the gold left in a completely coherent ribbon or 
plate). 

Instead of the term parting, the terms “quartation” or “inquartation” 
are sometimes used. 

7. Errors incidental to the processes described. Like all other practical 
operations, the processes described are more or less imperfect. 

In fusing in the crucible, minute quantities of silver and gold are 
volatilized in the heating operation of the crucible assay. The amount of 
loss from this source is of no consequence. (The fact of such loss is proved 
by an examination of the dust found in the chimney flues.) 

In cupelling, there may be a loss of silver by reason of the absorption of 
a minute amount of it—with oxide of lead into the cupel. 

There may be an apparent gain of silver, by reason of a small amount of 
copper remaining in the prill (this copper incidentally being reckoned later 
as silver). 

In parting, there may be a minute loss of gold by its solution (this dan¬ 
ger is the greater, if the nitric acid contains chlorine). 

There may be an apparent gain of gold, by reason of the fact that in¬ 
variably a small amount of silver—about 2 one-thousandths of the silver 
present—fails to dissolve in the parting operation. This amount is called 
the “surcharge”; it is usually “allowed for.” 

It is customary in government assaying establishments (and in others 
also) to learn what “allowances” should be made for gains or losses inci¬ 
dental to a particular method of assaying; this is accomplished by conduct¬ 
ing a precisely similar set of processes upon weighed amounts of pure silver, 
or of pure gold, or of known alloys of them 


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